doc-diff: use single-colon rule in rendering Makefile

When rendering the troff manpages to text via "man", we create an ad-hoc Makefile and feed it to "make". The purpose here is two-fold: - reuse results from a prior interrupted render of the same tree - use make's -j option to build in parallel But the second part doesn't seem to work (at least with my version of GNU make, 4.2.1). It just runs one render at a time. We use a double-colon "all" rule for each file, like: all:: foo foo: ...actual render recipe... all:: bar bar: ...actual render recipe... ...and so on... And it's this double-colon that seems to inhibit the parallelism. We can just switch to a regular single-colon rule. Even though we do have multiple rules for "all" here, we don't have any recipe to execute for "all" (we only care about triggering its dependencies), so the distinction is irrelevant. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>