format_commit_message(): fix function signature

The format template string was declared as "const void *" for some unknown
reason, even though it obviously is meant to be passed a string.  Make it
"const char *".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -70,7 +70,7 @@ extern char *reencode_commit_message(const struct commit *commit,
 				     const char **encoding_p);
 extern void get_commit_format(const char *arg, struct rev_info *);
 extern void format_commit_message(const struct commit *commit,
-				  const void *format, struct strbuf *sb,
+				  const char *format, struct strbuf *sb,
 				  enum date_mode dmode);
 extern void pretty_print_commit(enum cmit_fmt fmt, const struct commit*,
                                 struct strbuf *,