cli: detect bareness of git libraries

[jfontan]

Now by default the type of git repositories (bare or non bare) is chosen automatically using the first repository found in the directory. To force an specific format --bare and --non-bare can be used. Also the URL format now has 3 possible values for bare option: true, false and auto.

$ gitbase server --non-bare -d //dir/1
$ gitbase server -d 'file:///dir/1?bare=auto' -d 'file:///dir/1?bare=true'

• I updated the documentation explaining the new behavior if any.
• I updated CHANGELOG.md file adding the new feature or bug fix.
• I updated go-mysql-server using make upgrade command if applicable.
• I added or updated examples if applicable.
• I checked that changes on schema are reflected into the documentation, if applicable.

[jfontan]

I've rebased with latest master