The enter_repo() function is used to navigate into a .git
directory. It knows how to find standard alternatives (DWIM) but
it doesn't handle gitfiles created by git init --separate-git-dir.
This means that git-fetch and others do not work with repositories
using the separate-git-dir mechanism.
Teach enter_repo() to deal with the gitfile mechanism by resolving
the path to the redirected path and continuing tests on that path
instead of the found file.
Signed-off-by: Phil Hord <hordp@cisco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>