filter-branch: do not forget the '@' prefix to force git-timestamp

For some reason, this script reinvents, instead of refactoring the
existing one in git-sh-setup, the logic to grab ident information
from an existing commit; it was missed when the corresponding logic
in git-sh-setup was updated with 2c733fb2 (parse_date(): '@' prefix
forces git-timestamp, 2012-02-02).

Teach the script that it is OK to have a way ancient timestamp in
the commits that are being filtered.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -84,7 +84,7 @@ set_ident () {
 			s/.*/GIT_'$uid'_EMAIL='\''&'\''; export GIT_'$uid'_EMAIL/p
 
 			g
-			s/^'$lid' [^<]* <[^>]*> \(.*\)$/\1/
+			s/^'$lid' [^<]* <[^>]*> \(.*\)$/@\1/
 			s/'\''/'\''\'\'\''/g
 			s/.*/GIT_'$uid'_DATE='\''&'\''; export GIT_'$uid'_DATE/p