[git] Add git-tag-update

git/README

@@ -71,3 +71,19 @@ script in a directory in the PATH (for instance /usr/local/bin) and
 add an alias:
 
  git config --global alias.merge-ff-only "\!sh -c 'git-merge-ff-only'"
+
+
+# git-tag-update #
+
+git-tag-update allows one to update a tag message while automatically
+keeping its original date. It takes the name of the tag to update as an
+argument.
+
+Beware that the tag's author will be changed to whatever is the current
+author. You can change it temporarily with git config if you need to
+update commits with different authors.
+
+To activate the git tag-update command, put the git-tag-update script in
+a directory in the PATH (for instance /usr/local/bin) and add an alias:
+
+ git config --global alias.tag-update "\!sh -c 'git-tag-update'"

git/git-tag-update

@@ -0,0 +1,32 @@
+#!/bin/sh
+#
+# git-tag-update, Copyright © 2012 Matteo Cypriani
+#
+# This program is free software. It comes without any warranty, to
+# the extent permitted by applicable law. You can redistribute it
+# and/or modify it under the terms of the Do What The Fuck You Want
+# To Public License, Version 2, as published by Sam Hocevar. See
+# http://sam.zoy.org/wtfpl/COPYING for more details.
+#
+# This script allows one to update a Git tag message while keeping its
+# original date. The tag's author will be changed to whatever is the
+# current author (you can change it temporarily with git config if you
+# need to update commits with different authors).
+
+set -e
+#set -x
+
+if [ $# -ne 1 ] ; then
+	echo "Usage: $0 <tag-name>"
+	exit 1
+fi
+
+TAG=$1
+
+GIT_COMMITTER_DATE="$(git tag -v $TAG 2>/dev/null|grep ^tagger | cut -f5- -d' ')"
+export GIT_COMMITTER_DATE
+git tag -sf $TAG $TAG
+
+echo "Old tag commiter date: $GIT_COMMITTER_DATE"
+GIT_COMMITTER_DATE="$(git tag -v $TAG 2>/dev/null|grep ^tagger | cut -f5- -d' ')"
+echo "New tag commiter date: $GIT_COMMITTER_DATE"