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>

date.c

@@ -637,7 +637,8 @@ int parse_date_basic(const char *date, unsigned long *timestamp, int *offset)
 	*offset = -1;
 	tm_gmt = 0;
 
-	if (!match_object_header_date(date, timestamp, offset))
+	if (*date == '@' &&
+	    !match_object_header_date(date + 1, timestamp, offset))
 		return 0; /* success */
 	for (;;) {
 		int match = 0;