sparse: Fix an "symbol 'format_subject' not declared" warning

In order to fix the warning, we add an extern declaration for this
function to the "commit.h" header file, along with all other non-
static functions defined in pretty.c. Also, we remove the function
declaration from builtin/shortlog.c, since it is no longer needed.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/shortlog.c

@@ -29,9 +29,6 @@ static int compare_by_number(const void *a1, const void *a2)
 		return -1;
 }
 
-const char *format_subject(struct strbuf *sb, const char *msg,
-			   const char *line_separator);
-
 static void insert_one_record(struct shortlog *log,
 			      const char *author,
 			      const char *oneline)

commit.h

@@ -90,6 +90,8 @@ extern char *logmsg_reencode(const struct commit *commit,
 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 const char *format_subject(struct strbuf *sb, const char *msg,
+				  const char *line_separator);
 extern void userformat_find_requirements(const char *fmt, struct userformat_want *w);
 extern void format_commit_message(const struct commit *commit,
 				  const char *format, struct strbuf *sb,