[PATCH] Make git-mv work in subdirectories, too

[PATCH] Make git-mv work in subdirectories, too

Turns out, all git programs git-mv uses are capable of operating in
a subdirectory just fine. So don't complain about it.

[jc: I think that sounds sane.  You need to grab the exit status from
 `git-rev-parse --git-dir`, which I added.  Alex Riesen says this
 worked fine.]

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>