Make `magit-guess-branch' have git grep the reflog for at most one branch name.

This uses "git log --walk-reflogs --grep-reflog PATTERN", which is
supported in git >= 1.8.0.

I use magit to interact with mozilla-central (which is huge) and this change speeds things up dramatically.  (1-2 orders of magnitude, depending on disk cache.)

I see no feasible way to test this automatically.