git-bisect working only from toplevel dir

Hi!

The requirement to be in the toplevel directory when calling git-bisect is
pretty infuriating.  I tried to find an explanation for this, and the only
reference I found was:

http://thread.gmane.org/gmane.comp.version-control.git/27524/focus=27596

However, since then, git-reset has been changed (in a81c311f).  What about
changing git-bisect as well?

A trivial patch seems to work for me, but I might have missed some corner
case.

-- 
1KB		// Yo momma uses IPv4!