Add --no-commit-id option for git-diff-tree, use it in gitk

This patch introduces -no-commit-id option for git-diff-tree, which
suppresses commit ID output.

[jc: dropped gitk part for now.]

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-diff-tree.txt

@@ -8,7 +8,7 @@ git-diff-tree - Compares the content and mode of blobs found via two tree object
 
 SYNOPSIS
 --------
-'git-diff-tree' [--stdin] [-m] [-s] [-v] [--pretty] [-t] [-r] [--root] [<common diff options>] <tree-ish> [<tree-ish>] [<path>...]
+'git-diff-tree' [--stdin] [-m] [-s] [-v] [--no-commit-id] [--pretty] [-t] [-r] [--root] [<common diff options>] <tree-ish> [<tree-ish>] [<path>...]
 
 DESCRIPTION
 -----------
@@ -74,6 +74,10 @@ separated with a single space are given.
 	commit message.  Without "=<style>", it defaults to
 	medium.
 
+--no-commit-id::
+	git-diff-tree outputs a line with the commit ID when
+	applicable.  This flag suppressed the commit ID output.
+
 
 Limiting Output
 ---------------

diff-tree.c

@@ -3,6 +3,7 @@
 #include "commit.h"
 
 static int show_root_diff = 0;
+static int no_commit_id = 0;
 static int verbose_header = 0;
 static int ignore_merges = 1;
 static int read_stdin = 0;
@@ -29,7 +30,8 @@ static int call_diff_flush(void)
 		return 0;
 	}
 	if (header) {
-		printf("%s%c", header, diff_options.line_termination);
+		if (!no_commit_id)
+			printf("%s%c", header, diff_options.line_termination);
 		header = NULL;
 	}
 	diff_flush(&diff_options);
@@ -231,6 +233,10 @@ int main(int argc, const char **argv)
 			show_root_diff = 1;
 			continue;
 		}
+		if (!strcmp(arg, "--no-commit-id")) {
+			no_commit_id = 1;
+			continue;
+		}
 		usage(diff_tree_usage);
 	}
 	if (diff_options.output_format == DIFF_FORMAT_PATCH)