submodule: port resolve_relative_url from shell to C

Later on we want to automatically call `git submodule init` from
other commands, such that the users don't have to initialize the
submodule themselves.  As these other commands are written in C
already, we'd need the init functionality in C, too.  The
`resolve_relative_url` function is a large part of that init
functionality, so start by porting this function to C.

To create the tests in t0060, the function `resolve_relative_url`
was temporarily enhanced to write all inputs and output to disk
when running the test suite. The added tests in this patch are
a small selection thereof.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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