git-gui: Show the git-gui library path in 'About git-gui'

git-gui: Show the git-gui library path in 'About git-gui'

Because we now try to automatically guess the library directory
in certain installations users may wonder where git-gui is getting
its supporting files from.  We now display this location in our
About dialog, and we also include the location we are getting our
Git executables from.

Unfortunately users cannot use this 'About git-gui' dialog to
troubleshoot library loading problems; the dialog is defined by
code that exists in the library directory, creating a catch-22.

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