config: refactor include_by_gitdir() into include_by_path()

config: refactor include_by_gitdir() into include_by_path()

The include_by_gitdir() function matches the realpath of a given
path against a glob pattern, but its interface is tightly coupled to
the gitdir condition: it takes a struct config_options *opts and
extracts opts->git_dir internally.

Refactor it into a more generic include_by_path() helper that takes
a const char *path parameter directly, and update the gitdir and
gitdir/i callers to pass opts->git_dir explicitly.  No behavior
change, just preparing for the addition of a new worktree condition
that will reuse the same path-matching logic with a different path.

Signed-off-by: Chen Linxuan <me@black-desk.cn>
Signed-off-by: Junio C Hamano <gitster@pobox.com>