Simplify building git information for the repo segment.
One git command suffices for gathering all the information we need.
Use the --porcelain option for status, which exists to support tools
like this. Also pass -b to snag the branch and ahead/behind status.

We now get git info with one exec. Sadly, we're still perceptibly
slower than the python original. I suspect it's startup, but I'll have
to verify that.
And even more simplification of the git repo segment code path:
removed unnecessary variables & shortened condition detection.
Also, require only the modules we need when we need them.
Not helping the speed problem, though.