t/helper: 'test-chmtime (--get|-g)' to print only the mtime

t/helper: 'test-chmtime (--get|-g)' to print only the mtime

Compared to 'test-chmtime -v +0 file' which prints the mtime and
and the file name, 'test-chmtime --get file' displays only the mtime.
If it is used in combination with (+|=|=+|=-|-)seconds, it changes
and prints the new value.

	test-chmtime -v +0 file | sed 's/[^0-9].*$//'

is now equivalent to:

	test-chmtime --get file

Signed-off-by: Paul-Sebastian Ungureanu <redacted>
Signed-off-by: Junio C Hamano <redacted>