check-ref-format --print: Normalize refnames that start with slashes

check-ref-format --print: Normalize refnames that start with slashes

When asked if "refs///heads/master" is valid, check-ref-format says "Yes,
it is well formed", and when asked to print canonical form, it shows
"refs/heads/master". This is so that it can be tucked after "$GIT_DIR/"
to form a valid pathname for a loose ref, and we normalize a pathname like
"$GIT_DIR/refs///heads/master" to de-dup the slashes in it.

Similarly, when asked if "/refs/heads/master" is valid, check-ref-format
says "Yes, it is Ok", but the leading slash is not removed when printing,
leading to "$GIT_DIR//refs/heads/master".

Fix it to make sure such leading slashes are removed.  Add tests that such
refnames are accepted and normalized correctly.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>