parse_date(): '@' prefix forces git-timestamp

The only place that the issue this series addresses was observed
where we read "cat-file commit" output and put it in GIT_AUTHOR_DATE
in order to replay a commit with an ancient timestamp.

With the previous patch alone, "git commit --date='20100917 +0900'"
can be misinterpreted to mean an ancient timestamp, not September in
year 2010.  Guard this codepath by requring an extra '@' in front of
the raw git timestamp on the parsing side. This of course needs to
be compensated by updating get_author_ident_from_commit and the code
for "git commit --amend" to prepend '@' to the string read from the
existing commit in the GIT_AUTHOR_DATE environment variable.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-sh-setup.sh

@@ -200,7 +200,7 @@ get_author_ident_from_commit () {
 		s/.*/GIT_AUTHOR_EMAIL='\''&'\''/p
 
 		g
-		s/^author [^<]* <[^>]*> \(.*\)$/\1/
+		s/^author [^<]* <[^>]*> \(.*\)$/@\1/
 		s/.*/GIT_AUTHOR_DATE='\''&'\''/p
 
 		q