git-rerere: add 'gc' command.

Over time, unresolved rr-cache entries are accumulated and they
tend to get less and less likely to be useful as the tips of
branches advance.

Reorder documentation page to show the subcommand section earlier
than the discussion section.

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

Documentation/git-rerere.txt

@@ -26,6 +26,38 @@ results and applying the previously recorded hand resolution.
 You need to create `$GIT_DIR/rr-cache` directory to enable this
 command.
 
+
+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.
+
+'gc'::
+
+This command is used to prune records of conflicted merge that
+occurred long time ago.
+
+
 DISCUSSION
 ----------
 
@@ -166,30 +198,6 @@ 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

@@ -169,6 +169,28 @@ sub merge {
 	return 0;
 }
 
+sub garbage_collect_rerere {
+	# We should allow specifying these from the command line and
+	# that is why the caller gives @ARGV to us, but I am lazy.
+
+	my $cutoff_noresolve = 15; # two weeks
+	my $cutoff_resolve = 60; # two months
+	my @to_remove;
+	while (<$rr_dir/*/preimage>) {
+		my ($dir) = /^(.*)\/preimage$/;
+		my $cutoff = ((-f "$dir/postimage")
+			      ? $cutoff_resolve
+			      : $cutoff_noresolve);
+		my $age = -M "$_";
+		if ($cutoff <= $age) {
+			push @to_remove, $dir;
+		}
+	}
+	if (@to_remove) {
+		rmtree(\@to_remove);
+	}
+}
+
 -d "$rr_dir" || exit(0);
 
 read_rr();
@@ -198,6 +220,9 @@ sub merge {
 				"$rr_dir/$name/preimage", $path);
 		}
 	}
+	elsif ($arg eq 'gc') {
+		garbage_collect_rerere(@ARGV);
+	}
 	else {
 		die "$0 unknown command: $arg\n";
 	}