git-gui: Allow blame/browser subcommands on bare repositories

git-gui: Allow blame/browser subcommands on bare repositories

A long time ago Linus Torvalds tried to run git-gui on a bare
repository to look at the blame viewer, but it failed to start
because we required that the user run us only from within a
working directory that had a normal git repository associated
with it.

This change relaxes that requirement so that you can start the
tree browser or the blame viewer against a bare repository. In
the latter case we do require that you provide a revision and a
pathname if we cannot find the pathname in the current working
directory.

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