Add --verbose|-v to test-chmtime

This allows us replace perl when getting the mtime of a file because
of time zone conversions, though at the moment only one platform which
does this has been identified: Cygwin when used with ActiveState Perl
(as usual).

The output format is:

<mtime1> TAB <filename1> <LF>
<mtime2> TAB <filename2> <LF>
...

which, if only mtime is needed can be parsed with cut(1):

test-chmtime -v +0 filename1 | cut -f 1

Also, the change adds a description of programs features, with examples.

Signed-off-by: Alex Riesen <ariesen@harmanbecker.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>