remote: move relative_url()
This method was initially written in
63e95beb0 (submodule: port
resolve_relative_url from shell to C, 2016-05-15). As we will need
similar functionality in the bundle URI feature, extract this to be
available in remote.h.
The code is almost exactly the same, except for the following trivial
differences:
* Fix whitespace and wrapping issues with the prototype and argument
lists.
* Let's call starts_with_dot_{,dot_}slash_native() instead of the
functionally identical "starts_with_dot_{,dot_}slash()" wrappers
"builtin/submodule--helper.c".
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>