Add test-chmtime: a utility to change mtime on files

Add test-chmtime: a utility to change mtime on files

This is intended to be a portable replacement for our usage
of date(1), touch(1), and Perl one-liners in tests.

Usage: test-chtime (+|=|-|=+|=-)<seconds> <file>..."

  '+' increments the mtime on the files by <seconds>
  '-' decrements the mtime on the files by <seconds>
  '=' sets the mtime on the file to exactly <seconds>
  '=+' and '=-' sets the mtime on the file to <seconds> after or
      before the current time.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>