worktree move: new command

This command allows to relocate linked worktrees. Main worktree cannot
(yet) be moved.

There are two options to move the main worktree, but both have
complications, so it's not implemented yet. Anyway the options are:

- convert the main worktree to a linked one and move it away, leave
  the git repository where it is. The repo essentially becomes bare
  after this move.

- move the repository with the main worktree. The tricky part is make
  sure all file descriptors to the repository are closed, or it may
  fail on Windows.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -3087,7 +3087,7 @@ _git_whatchanged ()
 
 _git_worktree ()
 {
-	local subcommands="add list lock prune unlock"
+	local subcommands="add list lock move prune unlock"
 	local subcommand="$(__git_find_on_cmdline "$subcommands")"
 	if [ -z "$subcommand" ]; then
 		__gitcomp "$subcommands"