worktree: add top-level worktree.c

worktree.c contains functions to work with and get information from
worktrees.  This introduction moves functions related to worktrees
from branch.c into worktree.c

Signed-off-by: Michael Rappazzo <rappazzo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/notes.c b/builtin/notes.c
index 3608c64..13c0af9 100644
--- a/builtin/notes.c
+++ b/builtin/notes.c
@@ -19,7 +19,7 @@
 #include "string-list.h"
 #include "notes-merge.h"
 #include "notes-utils.h"
-#include "branch.h"
+#include "worktree.h"
 
 static const char * const git_notes_usage[] = {
 	N_("git notes [--ref <notes-ref>] [list [<object>]]"),