submodule: ignore trailing slash on superproject URL

submodule: ignore trailing slash on superproject URL

Before 63e95beb0 (2016-04-15, submodule: port resolve_relative_url from
shell to C), it did not matter if the superprojects URL had a trailing
slash or not. It was just chopped off as one of the first steps
(The "remoteurl=${remoteurl%/}" near the beginning of
resolve_relative_url(), which was removed in said commit).

When porting this to the C version, an off-by-one error was introduced
and we did not check the actual last character to be a slash, but the
NULL delimiter.

Reintroduce the behavior from before 63e95beb0, to ignore the trailing
slash.

Reported-by: <venv21@gmail.com>
Helped-by: Dennis Kaarsemaker <dennis@kaarsemaker.net>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>