Allow add_path() to add non-existent directories to the path

Allow add_path() to add non-existent directories to the path

This function had used make_absolute_path(); but this function dies if
the directory that contains the entry whose relative path was supplied in
the argument does not exist. This is a problem if the argument is, for
example, "../libexec/git-core", and that "../libexec" does not exist.

Since the resolution of symbolic links is not required for elements in
PATH, we can fall back to using make_nonrelative_path(), which simply
prepends $PWD to the path.

We have to move make_nonrelative_path() alongside make_absolute_path() in
abspath.c so that git-shell can be linked. See 5b8e6f85f.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>