Handle relative paths in submodule .git files

Handle relative paths in submodule .git files

Commit 842abf0 (Teach resolve_gitlink_ref() about the .git file, 2008-02-20)
taught resolve_gitlink_ref() to call read_gitfile_gently() to resolve .git
files.  In this commit teach read_gitfile_gently() to interpret a relative
path in a .git file with respect to the file location.

This change allows update-index to recognize a submodule that uses a relative
path in its .git file.  It previously failed because the relative path was
wrongly interpreted with respect to the superproject directory.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>