stripspace: allow -s/-c outside git repository

stripspace: allow -s/-c outside git repository

v2.11.0-rc3~3^2~1 (stripspace: respect repository config, 2016-11-21)
improved stripspace --strip-comments / --comentlines by teaching them
to read repository config, but it went a little too far: when running
stripspace outside any repository, the result is

	$ git stripspace --strip-comments <test-input
	fatal: not a git repository (or any parent up to mount point /tmp)

That makes experimenting with the stripspace command unnecessarily
fussy.  Fix it by discovering the git directory gently, as intended
all along.

Reported-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>