format-patch: wrap generate_commit_list_cover()

While most conventions should not allow for the text lines in commit
messages to get too long, when they do it could make emails harder to
read.

Teach generate_commit_list_cover() to wrap its commit lines if they
happend to be too long.

Signed-off-by: Mirko Faina <mroik@delayed.space>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Mroik

builtin/log.c

@@ -40,6 +40,7 @@
 #include "progress.h"
 #include "commit-slab.h"
 #include "advice.h"
+#include "utf8.h"
 
 #include "commit-reach.h"
 #include "range-diff.h"
@@ -1364,6 +1365,7 @@ static void generate_commit_list_cover(FILE *cover_file, const char *format,
 				       struct commit **list, int n)
 {
 	struct strbuf commit_line = STRBUF_INIT;
+	struct strbuf wrapped_line = STRBUF_INIT;
 	struct pretty_print_context ctx = {0};
 	struct rev_info rev = REV_INFO_INIT;
 
@@ -1373,12 +1375,16 @@ static void generate_commit_list_cover(FILE *cover_file, const char *format,
 		rev.nr = i;
 		repo_format_commit_message(the_repository, list[n - i], format,
 				&commit_line, &ctx);
-		fprintf(cover_file, "%s\n", commit_line.buf);
+		strbuf_add_wrapped_text(&wrapped_line, commit_line.buf, 0, 0,
+					MAIL_DEFAULT_WRAP);
+		fprintf(cover_file, "%s\n", wrapped_line.buf);
 		strbuf_reset(&commit_line);
+		strbuf_reset(&wrapped_line);
 	}
 	fprintf(cover_file, "\n");
 
 	strbuf_release(&commit_line);
+	strbuf_release(&wrapped_line);
 }
 
 static void make_cover_letter(struct rev_info *rev, int use_separate_file,