worktree: avoid dead-code in conditional

worktree: avoid dead-code in conditional

get_worktrees() retrieves a list of all worktrees associated with a
repository, including the main worktree. The location of the main
worktree is determined by get_main_worktree() which needs to handle
three distinct cases for the main worktree after absolute-path
conversion:

    * <bare-repository>/.
    * <main-worktree>/.git/. (when $CWD is .git)
    * <main-worktree>/.git (when $CWD is any worktree)

They all need to be normalized to just the <path> portion, dropping any
"/." or "/.git" suffix.

It turns out, however, that get_main_worktree() was only handling the
first and last cases, i.e.:

    if (!strip_suffix(path, "/.git"))
        strip_suffix(path, "/.");

This shortcoming was addressed by 45f274fbb1 (get_main_worktree(): allow
it to be called in the Git directory, 2020-02-23) by changing the logic
to:

    strip_suffix(path, "/.");
    if (!strip_suffix(path, "/.git"))
        strip_suffix(path, "/.");

which makes the final strip_suffix() invocation dead-code.

Fix this oversight by enumerating the three distinct cases explicitly
rather than attempting to strip the suffix(es) incrementally.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>