rerere: add clear, diff, and status commands

git-am and git-rebase will be updated to use 'clear', and
diff/status can be used to aid the user in tracking progress in
the resolution process.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-rerere.txt

@@ -7,8 +7,7 @@ git-rerere - Reuse recorded resolve
 
 SYNOPSIS
 --------
-'git-rerere'
-
+'git-rerere' [clear|diff|status]
 
 DESCRIPTION
 -----------
@@ -167,6 +166,30 @@ would conflict the same way the test merge you resolved earlier.
 `git-rerere` is run by `git rebase` to help you resolve this
 conflict.
 
+COMMANDS
+--------
+
+Normally, git-rerere is run without arguments or user-intervention.
+However, it has several commands that allow it to interact with
+its working state.
+
+'clear'::
+
+This resets the metadata used by rerere if a merge resolution is to be
+is aborted.  Calling gitlink:git-am[1] --skip or gitlink:git-rebase[1]
+[--skip|--abort] will automatcally invoke this command.
+
+'diff'::
+
+This displays diffs for the current state of the resolution.  It is
+useful for tracking what has changed while the user is resolving
+conflicts.  Additional arguments are passed directly to the system
+diff(1) command installed in PATH.
+
+'status'::
+
+Like diff, but this only prints the filenames that will be tracked
+for resolutions.
 
 Author
 ------

git-rerere.perl

@@ -172,6 +172,38 @@ sub merge {
 -d "$rr_dir" || exit(0);
 
 read_rr();
+
+if (@ARGV) {
+	my $arg = shift @ARGV;
+	if ($arg eq 'clear') {
+		for my $path (keys %merge_rr) {
+			my $name = $merge_rr{$path};
+			if (-d "$rr_dir/$name" &&
+			    ! -f "$rr_dir/$name/postimage") {
+				rmtree(["$rr_dir/$name"]);
+			}
+		}
+		unlink $merge_rr;
+	}
+	elsif ($arg eq 'status') {
+		for my $path (keys %merge_rr) {
+			print $path, "\n";
+		}
+	}
+	elsif ($arg eq 'diff') {
+		for my $path (keys %merge_rr) {
+			my $name = $merge_rr{$path};
+			system('diff', ((@ARGV == 0) ? ('-u') : @ARGV),
+				'-L', "a/$path", '-L', "b/$path",
+				"$rr_dir/$name/preimage", $path);
+		}
+	}
+	else {
+		die "$0 unknown command: $arg\n";
+	}
+	exit 0;
+}
+
 my %conflict = map { $_ => 1 } find_conflict();
 
 # MERGE_RR records paths with conflicts immediately after merge