git-gui: try harder to find worktree from gitdir

git-gui: try harder to find worktree from gitdir

git-gui, since 87cd09f43e ("git-gui: work from the .git dir",
2010-01-23), has had the intent to allow starting from inside a
repository, then switching to the parent directory if that is a valid
worktree.

This certainly hasn't worked since 2d92ab32fd ("rev-parse: make
--show-toplevel without a worktree an error", 2019-11-19) in git, but
breaking this git-gui feature was unintentional.

There are (at least) 3 cases where the gitdir can tell us where the
worktree is, and we would like all to work:

- core.worktree is set, and points to a valid worktree. This is already
  handled  by git rev-parse --show-toplevel, even when not in the worktree.
  There is nothing more to do in this case.

- the gitdir is embedded in a worktree as subdirectory .git. The parent
  is (or at least should be) a valid worktree. This worked long ago.

- the gitdir is a worktree specific directory (under
  <mainrepo>/worktrees/worktree_name), within which there is a file
  "gitdir" pointing to .git in the worktree. git gui never learned to
  handle this case.

Let's handle the latter two cases. Always check that the discovered
worktree is valid and points to the already discovered gitdir according
to git rev-parse. This avoids issues that may arise because we are
discovering from the gitdir up, rather than the worktree down, and file
system non-posix behavior or misconfiguration of git might cause
confusion.  For instance, a manually moved worktree might not be where
the gitdir points, or the gitdir might be configured with
core.bare=true.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>