worktree add: stop reading ".git/HEAD"

worktree add: stop reading ".git/HEAD"

The function can_use_local_refs() prints a warning if there are no local
branches and HEAD is invalid or points to an unborn branch. As part of
the warning it prints the contents of ".git/HEAD". In a repository using
the reftable backend HEAD is not stored in the filesystem so reading
that file is pointless. In a repository using the files backend it is
unclear how useful printing it is - it would be better to diagnose the
problem for the user. For now, simplify the warning by not printing
the file contents and adjust the relevant test case accordingly. Also
fixup the test case to use test_grep so that anyone trying to debug a
test failure in the future is not met by a wall of silence.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>