worktree: update is_bare heuristics

When "git branch -D <name>" is run, Git usually first checks if that
branch is currently checked out. But this check is not performed if the
Git directory of that repository is not at "<repo>/.git", which is the
case if that repository is a submodule that has its Git directory stored
as "super/.git/modules/<repo>", for example. This results in the branch
being deleted even though it is checked out.

This is because get_main_worktree() in worktree.c sets is_bare on a
worktree only using the heuristic that a repo is bare if the worktree's
path does not end in "/.git", and not bare otherwise. This is_bare code
was introduced in 92718b7438 ("worktree: add details to the worktree
struct", 2015-10-08), following a pre-core.bare heuristic. This patch
does 2 things:

 - Teach get_main_worktree() to use is_bare_repository() instead,
   introduced in 7d1864ce67 ("Introduce is_bare_repository() and
   core.bare configuration variable", 2007-01-07) and updated in
   e90fdc39b6 ("Clean up work-tree handling", 2007-08-01). This solves
   the "git branch -D <name>" problem described above. However...

 - If a repository has core.bare=1 but the "git" command is being run
   from one of its secondary worktrees, is_bare_repository() returns
   false (which is fine, since there is a worktree available). However,
   treating the main worktree as non-bare when it is bare causes issues:
   for example, failure to delete a branch from a secondary worktree
   that is referred to by a main worktree's HEAD, even if that main
   worktree is bare.

   In order to avoid that, also check core.bare when setting is_bare. If
   core.bare=1, trust it, and otherwise, use is_bare_repository().

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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