GitLab v320 git-annex fix

Hello,

I backported changes from git-annex branch of sitaramc/gitolite repo and also made the following changes on top to enable git-annex to work correctly with gitolite and GitLab:
- fixed processing of repository urls ending with .git (GitLab recommends urls like that to users by default),
- git-annex-shell command is added to .gitolite.rc (disabled by default)
- command pattern is changed to allow for some characters git-annex may include in its arguments

Best regards,
Oleg