test: add test cases for relative_path

test: add test cases for relative_path

Add subcommand "relative_path" in test-path-utils, and add test cases
in t0060.

Johannes tested an earlier version of this patch on Windows, and
found that some relative_path tests should be skipped on
Windows. This is because the bash on Windows rewrites arguments of
regular Windows programs, such as git and the test helpers, if the
arguments look like absolute POSIX paths. As a consequence, the
actual tests performed are not what the tests scripts expect.

The tests that need *not* be skipped are those where the two paths passed
to 'test-path-utils relative_path' have the same prefix and the result is
expected to be a relative path. This is because the rewriting changes
"/a/b" to "D:/Src/MSysGit/a/b", and when both inputs are extended the same
way, this just cancels out in the relative path computation.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>