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>

worktree.h

@@ -0,0 +1,12 @@
+#ifndef WORKTREE_H
+#define WORKTREE_H
+
+/*
+ * Check if a per-worktree symref points to a ref in the main worktree
+ * or any linked worktree, and return the path to the exising worktree
+ * if it is.  Returns NULL if there is no existing ref.  The caller is
+ * responsible for freeing the returned path.
+ */
+extern char *find_shared_symref(const char *symref, const char *target);
+
+#endif