format-patch: make zero-length subject prefixes prettier

If you give a zero-length subject prefix to format-patch
(e.g., "format-patch --subject-prefix="), we will print the
ugly:

  Subject: [ 1/2] your subject here

because we always insert a space between the prefix and
numbering. Requiring the user to provide the space in their
prefix would be more flexible, but would break existing
usage. This patch provides a DWIM and suppresses the space
for zero-length prefixes, under the assumption that nobody
actually wants "[ 1/2]".

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

log-tree.c

@@ -294,8 +294,9 @@ void log_write_email_headers(struct rev_info *opt, struct commit *commit,
 	if (opt->total > 0) {
 		static char buffer[64];
 		snprintf(buffer, sizeof(buffer),
-			 "Subject: [%s %0*d/%d] ",
+			 "Subject: [%s%s%0*d/%d] ",
 			 opt->subject_prefix,
+			 *opt->subject_prefix ? " " : "",
 			 digits_in_number(opt->total),
 			 opt->nr, opt->total);
 		subject = buffer;