git-gui: Don't offer my miga hack if its configuration file isn't present.

git-gui: Don't offer my miga hack if its configuration file isn't present.

I really hate that I have this specialized hack within git-gui, but
its here.  The hack shouldn't be offered unless miga's required .pvcsrc
file is in the top level of the repository's working directory.  If
this file is missing miga will fail to startup properly, and the user
cannot wouldn't be able to use it within this directory.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>