worktree: introduce is_shared_symref()

Add a new function, is_shared_symref(), which contains the heart of
find_shared_symref().  Refactor find_shared_symref() to use the new
function is_shared_symref().

Soon, we will use is_shared_symref() to search for symref beyond
the first worktree that matches.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

worktree.h

@@ -149,6 +149,12 @@ const struct worktree *find_shared_symref(struct worktree **worktrees,
 					  const char *symref,
 					  const char *target);
 
+/*
+ * Returns true if a symref points to a ref in a worktree.
+ */
+int is_shared_symref(const struct worktree *wt,
+		     const char *symref, const char *target);
+
 /*
  * Similar to head_ref() for all HEADs _except_ one from the current
  * worktree, which is covered by head_ref().