### Please describe the problem.

Our http://datasets.datalad.org has been providing git annex repos, some of which with the content, via a "dummy" HTTP support of git.  For various reasons (performance, progress reporting by git upon clone) we want to switch to use [Smart HTTP](https://git-scm.com/book/en/v2/Git-on-the-Server-Smart-HTTP) git-http-backend  backend.  Sample deployment is at http://datasets-dev.datalad.org/.
I followed the docs to set it up and only added one more configuration tune up


```
 RewriteEngine On

 RewriteCond  "%{HTTP_USER_AGENT}"  "(git)"

 RewriteRule ^(.*)$ "/git/$1" [PT]
```

so that people could still browse the website in the browser, but whenever `git` tries to access it, we direct to the `git-http-backend` CGI serving under `/git/` prefix (`ScriptAlias /git/ /usr/lib/git-core/git-http-backend/`).

Everything seems to work nicely on git side, BUT I am having difficulty to make git-annex being able to serve annexed files from it:
 
### What version of git-annex are you using? On what operating system?

6.20180913+git149-g23bd27773-1~ndall+1


### Please provide any additional information below.

[[!format sh """
$> builtin cd /tmp/; rm -rf raiders; git clone http://datasets-dev.datalad.org/labs/haxby/raiders/ ; cd raiders; git annex get sub-rid000005/anat/sub-rid000005_run-01_T1w_defacemask.nii.gz                                                                                                                          Cloning into 'raiders'...                
remote: Counting objects: 17926, done.
remote: Compressing objects: 100% (7203/7203), done.
remote: Total 17926 (delta 7356), reused 15517 (delta 6237)
Receiving objects: 100% (17926/17926), 1.23 MiB | 6.53 MiB/s, done.
Resolving deltas: 100% (7356/7356), done.
README.md                 masks/       stimulus/       sub-rid000014/  sub-rid000028/  sub-rid000038/  task-raiders_bold.json
dataset_description.json  scripts/     sub-rid000005/  sub-rid000015/  sub-rid000029/  sub-rid000042/
derivatives/              sourcedata/  sub-rid000011/  sub-rid000020/  sub-rid000033/  sub-rid000043/
(merging origin/git-annex into git-annex...)
(recording state in git...)
get sub-rid000005/anat/sub-rid000005_run-01_T1w_defacemask.nii.gz download failed: Not Found

  Remote origin not usable by git-annex; setting annex-ignore
(not available) 
  Try making some of these repositories available:
  	41e5039d-1750-43d2-8bea-89897d969326 -- /mnt/datasets/datalad/crawl/labs/haxby/raiders
   	87d7db62-683d-43b2-b594-baeb420ae7a6 -- .
   	afde6679-1f2f-41f2-935a-93e7e3d70274 -- nastase@head1:~/BIDS/haxby/raiders
   	de53ce43-2c07-4971-8de8-0445c596f7dc -- datalad-public-ro

  (Note that these git remotes have annex-ignore set: origin)
failed
(recording state in git...)
git-annex: get: 1 failed
"""]]

fails because `config` file is under `.git/` subdirectory there and git-annex doesn't try to access it at all to deduce the uuid, thus marking origin as annex-ignore.

But if I add that `.git` suffix to the url, then:

[[!format sh """
(git)hopa:/tmp/raiders[master]
$> builtin cd /tmp/; rm -rf raiders; git clone http://datasets-dev.datalad.org/labs/haxby/raiders/.git/ ; cd raiders; git annex get sub-rid000005/anat/sub-rid000005_run-01_T1w_defacemask.nii.gz                                                                 
Cloning into 'raiders'...
remote: Counting objects: 17926, done.
remote: Compressing objects: 100% (7203/7203), done.
remote: Total 17926 (delta 7356), reused 15517 (delta 6237)
Receiving objects: 100% (17926/17926), 1.23 MiB | 5.08 MiB/s, done.
Resolving deltas: 100% (7356/7356), done.
README.md                 masks/       stimulus/       sub-rid000014/  sub-rid000028/  sub-rid000038/  task-raiders_bold.json
dataset_description.json  scripts/     sub-rid000005/  sub-rid000015/  sub-rid000029/  sub-rid000042/
derivatives/              sourcedata/  sub-rid000011/  sub-rid000020/  sub-rid000033/  sub-rid000043/
(merging origin/git-annex into git-annex...)
(recording state in git...)
get sub-rid000005/anat/sub-rid000005_run-01_T1w_defacemask.nii.gz (from origin...) 
download failed: Not Found
download failed: Not Found

  Unable to access these remotes: origin

  Try making some of these repositories available:
  	41e5039d-1750-43d2-8bea-89897d969326 -- /mnt/datasets/datalad/crawl/labs/haxby/raiders
   	87d7db62-683d-43b2-b594-baeb420ae7a6 -- .
   	afde6679-1f2f-41f2-935a-93e7e3d70274 -- nastase@head1:~/BIDS/haxby/raiders
   	de53ce43-2c07-4971-8de8-0445c596f7dc -- datalad-public-ro [origin]
failed
(recording state in git...)
git-annex: get: 1 failed
"""]]
because it fails to find those two files under `.git/annex/objects`, here is apache log file


```
10.31.191.134 - - [01/Oct/2018:13:01:58 -0400] "GET /labs/haxby/raiders/.git//config HTTP/1.1" 206 501 "-" "-"

10.31.191.134 - - [01/Oct/2018:13:01:58 -0400] "GET /labs/haxby/raiders/.git//annex/objects/Z8/f1/MD5E-s41438--06c245e709e7d40a90ed48c6c3b58295.nii.gz/MD5E-s41438--06c245e709e7d40a90ed48c6c3b58295.nii.gz HTTP/1.1" 404 243 "-" "git-annex/6.20180913+git149-g23bd27773-1~ndall+1"

10.31.191.134 - - [01/Oct/2018:13:01:58 -0400] "GET /labs/haxby/raiders/.git//annex/objects/681/5d0/MD5E-s41438--06c245e709e7d40a90ed48c6c3b58295.nii.gz/MD5E-s41438--06c245e709e7d40a90ed48c6c3b58295.nii.gz HTTP/1.1" 404 243 "-" "git-annex/6.20180913+git149-g23bd27773-1~ndall+1"
```

where it seems to assume different layout:

[[!format sh """
$> ls -dl $webroot//labs/haxby/raiders/.git/annex/objects/*/*/MD5E-s41438--06c245e709e7d40a90ed48c6c3b58295.nii.gz
drwxrwsr-x 1 yoh datalad 104 Sep 26  2016 /mnt/btrfs/manual-snapshots/srv-20180928/datasets.datalad.org/www///labs/haxby/raiders/.git/annex/objects/Z8/f1/MD5E-s41438--06c245e709e7d40a90ed48c6c3b58295.nii.gz/
"""]]


which git-annex assumes when working with the dummy HTTP:

```
10.31.191.134 - - [01/Oct/2018:13:09:53 -0400] "GET /labs/haxby/raiders/.git//config HTTP/1.1" 206 501 "-" "-"

10.31.191.134 - - [01/Oct/2018:13:09:53 -0400] "GET /labs/haxby/raiders/.git//annex/objects/Z8/f1/MD5E-s41438--06c245e709e7d40a90ed48c6c3b58295.nii.gz/MD5E-s41438--06c245e709e7d40a90ed48c6c3b58295.nii.gz HTTP/1.1" 200 41679 "-" "git-annex/6.20180913+git149-g23bd27773-1~ndall+1"
```

So I wonder if I need to do something on my end in configuring apache2, or something could/should be done on git-annex side?  Ideally I would like to be able to just clone them without specifying `.git/` suffix to the url.

But also note that `git-annex` seems to not even provide any agent value while trying to access `config` file:

```
10.31.191.134 - - [01/Oct/2018:13:12:45 -0400] "GET /labs/haxby/raiders/.git//config HTTP/1.1" 206 501 "-" "-"
```
[[fixed|done]] --[[Yarik]]
