worktree: add utility to find worktree by pathname

find_worktree() employs heuristics to match user provided input -- which
may be a pathname or some sort of shorthand -- with an actual worktree.
Although this convenience allows a user to identify a worktree with
minimal typing, the black-box nature of these heuristics makes it
potentially difficult for callers which already know the exact path of a
worktree to be confident that the correct worktree will be returned for
any specific pathname (particularly a relative one), especially as the
heuristics are enhanced and updated.

Therefore, add a companion function, find_worktree_by_path(), which
deterministically identifies a worktree strictly by pathname with no
interpretation and no magic matching.

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

worktree.h

@@ -61,6 +61,12 @@ struct worktree *find_worktree(struct worktree **list,
 			       const char *prefix,
 			       const char *arg);
 
+/*
+ * Return the worktree corresponding to `path`, or NULL if no such worktree
+ * exists.
+ */
+struct worktree *find_worktree_by_path(struct worktree **, const char *path);
+
 /*
  * Return true if the given worktree is the main one.
  */