worktree: generalize candidate worktree path validation

worktree: generalize candidate worktree path validation

"git worktree add" checks that the specified path is a valid location
for a new worktree by ensuring that the path does not already exist and
is not already registered to another worktree (a path can be registered
but missing, for instance, if it resides on removable media). Since "git
worktree add" is not the only command which should perform such
validation ("git worktree move" ought to also), generalize the the
validation function for use by other callers, as well.

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