Enchance git support to show tags and ongoing operations

[PedroWitzel]

The git support is limited, probably by design to keep it short, but coming from git bash, I found there was some vital information missing, like:

• the tag value when checkout on one,
• the commit SHA1 on the same case,
• Rebase, cherry-picking, ..., information (reference branch, commits already treated and total)

[PedroWitzel]

For more context: I've recently installed fish and I've tested a couple of themes and this looked particularly good. The error code and CMD time specially 👍

I've already ported some functionalities from git-prompt.sh on my fork.

Let me know if this interest you and what else can be done (i.e.: pushing what I've done under a themes_display_git_enhanced)