revision: drop useless string offset when parsing "--pretty"

Once upon a time, we parsed pretty options by looking for
"--pretty" at the start of the string, and then feeding the
rest (including an "=") to get_commit_format. Later, commit
48ded91 (log --pretty: do not accept bogus "--prettyshort",
2008-05-25) split this into a separate check for "--pretty"
versus "--pretty=".

However, when parsing "--pretty", we still passed "arg+8" to
get_commit_format. This is useless, since it will always
point to the NUL terminator at the end of the string. We can
simply pass NULL instead; both parameters are treated the
same by get_commit_format.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -1825,7 +1825,7 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 	} else if (!strcmp(arg, "--pretty")) {
 		revs->verbose_header = 1;
 		revs->pretty_given = 1;
-		get_commit_format(arg+8, revs);
+		get_commit_format(NULL, revs);
 	} else if (starts_with(arg, "--pretty=") || starts_with(arg, "--format=")) {
 		/*
 		 * Detached form ("--pretty X" as opposed to "--pretty=X")