combine-diff: a few more finishing touches.

"new file" and "deleted file" were already reported in the
original code, but the logic was not as transparent as it could
have.  This uses a few variables and more comments to clarify
the flow.  The rule is: (1) if a path exists in the merge result
when no parent had it, we report "new" (otherwise it came from
the parents, as opposed to have added by the evil merge). (2) if
the path does not exist in the merge result, it is "deleted".

Since we can say "new" and "deleted", there is no reason not to
follow the /dev/null convention.  This fixes it.

Appending function name after @@@ ... @@@ is trivial, so
implement it.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/diff-format.txt

@@ -212,9 +212,9 @@ copying detection) are designed to work with diff of two
        --- a/file
        +++ b/file
 +
-Contrary to two-line header for traditional 'unified' diff
-format, and similar to filenames in ordinary "diff header",
-/dev/null is not used for creation or deletion.
+Similar to two-line header for traditional 'unified' diff
+format, `/dev/null` is used to signal created or deleted
+files.
 
 4.   Chunk header format is modified to prevent people from
      accidentally feeding it to `patch -p1`. Combined diff format

combine-diff.c

@@ -489,6 +489,12 @@ static void show_parent_lno(struct sline *sline, unsigned long l0, unsigned long
 	printf(" -%lu,%lu", l0, l1-l0);
 }
 
+static int hunk_comment_line(const char *bol)
+{
+	int ch = *bol & 0xff;
+	return (isalpha(ch) || ch == '_' || ch == '$');
+}
+
 static void dump_sline(struct sline *sline, unsigned long cnt, int num_parent,
 		       int use_color)
 {
@@ -508,8 +514,13 @@ static void dump_sline(struct sline *sline, unsigned long cnt, int num_parent,
 		struct sline *sl = &sline[lno];
 		unsigned long hunk_end;
 		unsigned long rlines;
-		while (lno <= cnt && !(sline[lno].flag & mark))
+		const char *hunk_comment = NULL;
+
+		while (lno <= cnt && !(sline[lno].flag & mark)) {
+			if (hunk_comment_line(sline[lno].bol))
+				hunk_comment = sline[lno].bol;
 			lno++;
+		}
 		if (cnt < lno)
 			break;
 		else {
@@ -526,6 +537,22 @@ static void dump_sline(struct sline *sline, unsigned long cnt, int num_parent,
 			show_parent_lno(sline, lno, hunk_end, i);
 		printf(" +%lu,%lu ", lno+1, rlines);
 		for (i = 0; i <= num_parent; i++) putchar(combine_marker);
+
+		if (hunk_comment) {
+			int comment_end = 0;
+			for (i = 0; i < 40; i++) {
+				int ch = hunk_comment[i] & 0xff;
+				if (!ch || ch == '\n')
+					break;
+				if (!isspace(ch))
+				    comment_end = i;
+			}
+			if (comment_end)
+				putchar(' ');
+			for (i = 0; i < comment_end; i++)
+				putchar(hunk_comment[i]);
+		}
+
 		printf("%s\n", c_reset);
 		while (lno < hunk_end) {
 			struct lline *ll;
@@ -707,6 +734,8 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 		int use_color = opt->color_diff;
 		const char *c_meta = diff_get_color(use_color, DIFF_METAINFO);
 		const char *c_reset = diff_get_color(use_color, DIFF_RESET);
+		int added = 0;
+		int deleted = 0;
 
 		if (rev->loginfo)
 			show_log(rev, opt->msg_sep);
@@ -722,7 +751,10 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 		printf("..%s%s\n", abb, c_reset);
 
 		if (mode_differs) {
-			int added = !!elem->mode;
+			deleted = !elem->mode;
+
+			/* We say it was added if nobody had it */
+			added = !deleted;
 			for (i = 0; added && i < num_parent; i++)
 				if (elem->parent[i].status !=
 				    DIFF_STATUS_ADDED)
@@ -731,7 +763,7 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 				printf("%snew file mode %06o",
 				       c_meta, elem->mode);
 			else {
-				if (!elem->mode)
+				if (deleted)
 					printf("%sdeleted file ", c_meta);
 				printf("mode ");
 				for (i = 0; i < num_parent; i++) {
@@ -743,8 +775,14 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 			}
 			printf("%s\n", c_reset);
 		}
-		dump_quoted_path("--- a/", elem->path, c_meta, c_reset);
-		dump_quoted_path("+++ b/", elem->path, c_meta, c_reset);
+		if (added)
+			dump_quoted_path("--- /dev/", "null", c_meta, c_reset);
+		else
+			dump_quoted_path("--- a/", elem->path, c_meta, c_reset);
+		if (deleted)
+			dump_quoted_path("+++ /dev/", "null", c_meta, c_reset);
+		else
+			dump_quoted_path("+++ b/", elem->path, c_meta, c_reset);
 		dump_sline(sline, cnt, num_parent, opt->color_diff);
 	}
 	free(result);