Don't modify CREDITS-FILE if it hasn't changed.

Don't modify CREDITS-FILE if it hasn't changed.

We should always avoid rewriting a built file during `make install`
if nothing has changed since `make all`.  This is to help support
the typical installation process of compiling a package as yourself,
then installing it as root.

Forcing CREDITS-FILE to be always be rebuilt in the Makefile means
that CREDITS-GEN needs to check for a change and only update
CREDITS-FILE if the file content actually differs.  After all,
content is king in Git.

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