checkout bugfix: use stat.mtime instead of stat.ctime in two places

checkout bugfix: use stat.mtime instead of stat.ctime in two places

Commit e1afca4fd "write_index(): update index_state->timestamp after
flushing to disk" on 2009-02-23 used stat.ctime to record the
timestamp of the index-file.  This is wrong, so fix this and use the
correct stat.mtime timestamp instead.

Commit 110c46a909 "Not all systems use st_[cm]tim field for ns
resolution file timestamp" on 2009-03-08, has a similar bug for the
builtin-fetch-pack.c file.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>