Support gitconfig in XDG directory

[guyhughes]

Git began fully implementing support for config files in $XDG_CONFIG_HOME fairly recently in v2.4.3. Here's the commit:
git/git@ea19289

Using the XDG directory, the git-config supported path is $XDG_CONFIG_HOME/git/config, and if the environment variable is undefined it ought to default to $HOME/.config.

@m040601 requested this feature.
#63 (comment)

I'd be interested in implementing.

[guyzmo]

ok, two tasks are needed:

• check and avoid any reference to $HOME/.gitconfig in the sources
• have the gitpython-developers/GitPython abstraction provide us with the right path.

what I believe is that it's not git-repo's role to decide where the configuration is.

[guyzmo]

I can see there's a facility in gitpython to provide .gitconfig path.

I believe providing the proper path shouldn't be hard.

[guyzmo]

looking further, the impact seems to located mostly in two use cases of the application:

• when working outside of a repository, then the .gitconfig path is resolved using a hardcoded $HOME/.gitconfig, otherwise it uses gitpython to resolve the full configuration of the current repository ;
• when running git-repo config which defaults to '$HOME/.gitconfig', but can be overridden with the --config option.

[guyzmo]

sorry for not having left you the change to do it, but I went over that part of the code and just did it.
Though it's not a thorough implementation and it'll ignore the system part. It's just a hardcoded precedence. It might be improvable, but rather 👌 as is 😉

Please test the devel branch, I'm going to release it early next week (I'm waiting on new release of pybitbucket and that FOSDEM is over… whichever comes first 😉)