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>