Implement wrap format %w() as if it is a mode switch

I always considered line wrapping to be more similar to a colour, i.e. a
state that one can change and that is applied to all following text until
the next state change, except that it's always reset at the end of the
format string.

Here's a patch to implement this behaviour, using Dscho's
strbuf_add_wrapped_text()

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

pretty.c

@@ -445,6 +445,7 @@ struct format_commit_context {
 	enum date_mode dmode;
 	unsigned commit_header_parsed:1;
 	unsigned commit_message_parsed:1;
+	size_t width, indent1, indent2;
 
 	/* These offsets are relative to the start of the commit message. */
 	struct chunk author;
@@ -458,6 +459,7 @@ struct format_commit_context {
 	struct chunk abbrev_commit_hash;
 	struct chunk abbrev_tree_hash;
 	struct chunk abbrev_parent_hashes;
+	size_t wrap_start;
 };
 
 static int add_again(struct strbuf *sb, struct chunk *chunk)
@@ -595,6 +597,35 @@ static void format_decoration(struct strbuf *sb, const struct commit *commit)
 		strbuf_addch(sb, ')');
 }
 
+static void strbuf_wrap(struct strbuf *sb, size_t pos,
+			size_t width, size_t indent1, size_t indent2)
+{
+	struct strbuf tmp = STRBUF_INIT;
+
+	if (pos)
+		strbuf_add(&tmp, sb->buf, pos);
+	strbuf_add_wrapped_text(&tmp, sb->buf + pos,
+				(int) indent1, (int) indent2, (int) width);
+	strbuf_swap(&tmp, sb);
+	strbuf_release(&tmp);
+}
+
+static void rewrap_message_tail(struct strbuf *sb,
+				struct format_commit_context *c,
+				size_t new_width, size_t new_indent1,
+				size_t new_indent2)
+{
+	if (c->width == new_width && c->indent1 == new_indent1 &&
+	    c->indent2 == new_indent2)
+		return;
+	if (c->wrap_start && c->wrap_start < sb->len)
+		strbuf_wrap(sb, c->wrap_start, c->width, c->indent1, c->indent2);
+	c->wrap_start = sb->len;
+	c->width = new_width;
+	c->indent1 = new_indent1;
+	c->indent2 = new_indent2;
+}
+
 static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
                                void *context)
 {
@@ -645,6 +676,30 @@ static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
 			return 3;
 		} else
 			return 0;
+	case 'w':
+		if (placeholder[1] == '(') {
+			unsigned long width = 0, indent1 = 0, indent2 = 0;
+			char *next;
+			const char *start = placeholder + 2;
+			const char *end = strchr(start, ')');
+			if (!end)
+				return 0;
+			if (end > start) {
+				width = strtoul(start, &next, 10);
+				if (*next == ',') {
+					indent1 = strtoul(next + 1, &next, 10);
+					if (*next == ',') {
+						indent2 = strtoul(next + 1,
+								 &next, 10);
+					}
+				}
+				if (*next != ')')
+					return 0;
+			}
+			rewrap_message_tail(sb, c, width, indent1, indent2);
+			return end - placeholder + 1;
+		} else
+			return 0;
 	}
 
 	/* these depend on the commit */
@@ -748,7 +803,9 @@ void format_commit_message(const struct commit *commit,
 	memset(&context, 0, sizeof(context));
 	context.commit = commit;
 	context.dmode = dmode;
+	context.wrap_start = sb->len;
 	strbuf_expand(sb, format, format_commit_item, &context);
+	rewrap_message_tail(sb, &context, 0, 0, 0);
 }
 
 static void pp_header(enum cmit_fmt fmt,