pretty.c: add %f format specifier to format_commit_message()

This specifier represents the sanitized and filename friendly subject
line of a commit. No checks are made against the length of the string,
so users may need to trim the result to the desired length if using as a
filename. This is commonly used by format-patch to massage commit
subjects into filenames and output patches to files.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -121,6 +121,7 @@ The placeholders are:
 - '%d': ref names, like the --decorate option of linkgit:git-log[1]
 - '%e': encoding
 - '%s': subject
+- '%f': sanitized subject line, suitable for a filename
 - '%b': body
 - '%Cred': switch color to red
 - '%Cgreen': switch color to green

pretty.c

@@ -493,6 +493,38 @@ static void parse_commit_header(struct format_commit_context *context)
 	context->commit_header_parsed = 1;
 }
 
+static int istitlechar(char c)
+{
+	return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') ||
+		(c >= '0' && c <= '9') || c == '.' || c == '_';
+}
+
+static void format_sanitized_subject(struct strbuf *sb, const char *msg)
+{
+	size_t trimlen;
+	int space = 2;
+
+	for (; *msg && *msg != '\n'; msg++) {
+		if (istitlechar(*msg)) {
+			if (space == 1)
+				strbuf_addch(sb, '-');
+			space = 0;
+			strbuf_addch(sb, *msg);
+			if (*msg == '.')
+				while (*(msg+1) == '.')
+					msg++;
+		} else
+			space |= 1;
+	}
+
+	/* trim any trailing '.' or '-' characters */
+	trimlen = 0;
+	while (sb->buf[sb->len - 1 - trimlen] == '.'
+		|| sb->buf[sb->len - 1 - trimlen] == '-')
+		trimlen++;
+	strbuf_remove(sb, sb->len - trimlen, trimlen);
+}
+
 const char *format_subject(struct strbuf *sb, const char *msg,
 			   const char *line_separator)
 {
@@ -683,6 +715,9 @@ static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
 	case 's':	/* subject */
 		format_subject(sb, msg + c->subject_off, " ");
 		return 1;
+	case 'f':	/* sanitized subject */
+		format_sanitized_subject(sb, msg + c->subject_off);
+		return 1;
 	case 'b':	/* body */
 		strbuf_addstr(sb, msg + c->body_off);
 		return 1;