http-push: Normalise directory names when pushing to some WebDAV servers

http-push: Normalise directory names when pushing to some WebDAV servers

Fix a bug when pushing to WebDAV servers which do not use a trailing
slash for collection names. The previous implementation fails to see
that the requested resource "refs/" is the same resource as "refs"
and loads every reference twice (once for refs/ and once for refs).

This implementation normalises every collection name by appending a
trailing slash if necessary.

This can be tested with old versions of Apache (such as the WebDAV
server of GMX, Apache 2.0.63).

Based-on-patch-by: Gabriel Corona <gabriel.corona@enst-bretagne.fr>
Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>