git-gui: Allow users to browse any branch, not just the current one

git-gui: Allow users to browse any branch, not just the current one

We now allow users to pick which commit they want to browse through
our revision picking mega-widget.  This opens up in a dialog first,
and then opens a tree browser for that selected commit.  It is a very
simple approach and requires minimal code changes.

I also clarified the language a bit in the Repository menu, to show
that these actions will access files.  Just in case a user is not
quite sure what specific action they are looking for, but they know
they want some sort of file thing.

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