gitrepo: fix argument names for getremotechanges()
The argument name was changed from revs to heads in commit
mercurial-devel@8a0fca925992, included in Mercurial 1.9.
Interestingly enough, the change that actually added this name,
2dcfd4bbfc1a, was made 20 days after the actual rename, but before it
was released.
Looks like this is a very old bug
Fixes: #392 (closed)