git-gui: Tighten internal pattern match for lib/ directory

git-gui: Tighten internal pattern match for lib/ directory

Our GITGUI_LIBDIR macro was testing only for @@ at the start of
the path, assuming nobody would ever find that to be a reasonable
prefix for a directory to install our library into.  That is most
likely a valid assumption, but its even more unlikely they would
have the start be @@GITGUI_ and the end be @@.  Note that we
cannot use the full string here because that would get expanded
by the sed replacement in our Makefile.

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