dl-wrapper: Fix support for URIs containing '+'

dl-wrapper: Fix support for URIs containing '+'

'+' is a valid character in a url. The current dl-wrapper gets the
URI scheme by dropping everything after the last '+' character, with
the intension of finding 'git' from e.g. 'git+https://uri'.

If a uri has a '+' anywhere in it, it ends up using too much of the
string as a scheme, and fails to match the handler properly.

An example of where this form of URI is used is when using deploy tokens
in gitlab. It uses a form like https://<username>:<password>@gitlab.com/<group>/<repo.git>
where username for deploy token is of the form 'gitlab+deploy-token-<number>'.

Use the %% operator to search backwards until the last '+' character when
dropping the rest of the string as we know that the first '+'
in the string should be the scheme.

Signed-off-by: Robert Beckett <bbeckett@netvu.org.uk>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>