Include git-gui credits file in dist.

The Makefile for the git-gui subproject will fail to execute if run
outside of a git.git directory, such as when building from a .tar.gz
or .tar.bz2.  This is because it is looking for the credits file,
which was created but omitted from the tarball by the toplevel
Makefile.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -895,7 +895,8 @@ dist: git.spec git-archive
 	$(TAR) rf $(GIT_TARNAME).tar \
 		$(GIT_TARNAME)/git.spec \
 		$(GIT_TARNAME)/version \
-		$(GIT_TARNAME)/git-gui/version
+		$(GIT_TARNAME)/git-gui/version \
+		$(GIT_TARNAME)/git-gui/credits
 	@rm -rf $(GIT_TARNAME)
 	gzip -f -9 $(GIT_TARNAME).tar