Call strftime() in context of committer's TZ.

The initial patch for calculating the fast-export timestamp using the
TZ field of the authors file was broken.  strftime() should have been
called in the context of the committer's timezone along with
localtime().

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Fix authors map when -C <git_repository> is specified.

Signed-off-by: Chris Rorvick <chris@rorvick.com>