config: use gitdir to get worktree config

config: use gitdir to get worktree config

Update 'do_git_config_sequence()' to read the worktree config from
'config.worktree' in 'opts->git_dir' rather than the gitdir of
'the_repository'.

The worktree config is loaded from the path returned by
'git_pathdup("config.worktree")', the 'config.worktree' relative to the
gitdir of 'the_repository'. If loading the config for a submodule, this path
is incorrect, since 'the_repository' is the superproject. 'opts->git_dir' is
the gitdir of the submodule being configured, so the config file in that
location should be read instead.

To ensure the use of 'opts->git_dir' is safe, require that 'opts->git_dir'
is set if-and-only-if 'opts->commondir' is set (rather than "only-if" as it
is now). In all current usage of 'config_options', these values are set
together, so the stricter check does not change any behavior.

Finally, add tests to 't3007-ls-files-recurse-submodules.sh' to verify the
corrected config is loaded. Use 'ls-files' to test this because, unlike some
other '--recurse-submodules' commands, 'ls-files' parses the config of the
submodule in the same process as the superproject (via 'show_submodule()' ->
'repo_read_index()' -> 'prepare_repo_settings()'). As a result,
'the_repository' points to the config of the superproject but the
commondir/gitdir in the config sequence will be that of the submodule,
providing the exact scenario needed to verify this patch.

The first test ('--recurse-submodules parses submodule repo config') checks
that the submodule's *repo* config is read when running 'ls-files' on the
superproject; this confirms already-working behavior, serving as a reference
for how worktree config parsing should behave. The second test
('--recurse-submodules parses submodule worktree config') tests the same
scenario as the previous but instead using the *worktree* config,
demonstrating the corrected behavior. The 'test_config' helper is extended
for this case so that it properly applies the '--worktree' option to the
configure/unconfigure operations it performs.

Note that, although the submodule worktree config is now parsed instead of
the superproject's, 'extensions.worktreeConfig' in the superproject still
controls whether or not the worktree config is enabled at all in the
submodule. This will be fixed in a later patch.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>