[kexec] The version info grabber now groks git.

[John Stumpo]

In master, I have made the version info grabber (revtag/revtag.py) 
actually have an idea as to what git is.  So now a straight tree of 
master should actually build.

(It also will be perfectly happy in the absence of a git repository, 
which is really the right thing to have it do.  It's long past time that 
I made it do that, but I wrote the script about a year and a half ago 
and didn't ever feel like doing so.)

So now we can move on to more important things, like, say, figuring out 
those interrupt delivery issues.

Cheers,
John