Add an option to display the current git branch only.

[laurentbardin]

This is a first draft fix for #399. It has a few drawbacks and could be enhanced if needed:

1. if we wanted to implement the mechanism for every supported VCS, it would add one variable each time;
2. we could use one variable for every VCS, something like LP_ENABLE_VCS_BRANCH_ONLY, but then we lose fine-grained control over it.

I don't think that second point really matters, as the problem mentioned in the issue (slow I/O and big repo) is bound to happen with every VCS at some point.

As for the patch itself, I'm really not fluent with bash so feel free to point any mistake. It looks messy, but it's mostly changes in indentation.

[laurentbardin]

Fixed a conflict introduced by #508.

[dolmen]

This implementation is not efficient. This is the whole function definition that should be overriden if LP_ENABLE_GIT_BRANCH_ONLY is set.

[Rycieos]

Release Candidate v2.0.0-rc.1 is now out, which means that the rework is complete. This has merge conflicts that need to be fixed.

This also is something that should be handled by a Liquidprompt theme. The included alternate_vcs theme is similar to what you are trying to do. I added a barebones example theme to the linked issue.

But if you think others could benefit from this, you can open a PR (or update this one) with a theme in themes/ similar to the alternate_vcs theme. And feel free to ask for help!

[Rycieos]

Closing as this needs work, a rebase, should now be done with themes, and there have been no updates for years.