wt-status: tolerate dangling marks

When a user checks out the upstream branch of HEAD, the upstream branch
not being a local branch, and then runs "git status", like this:

  git clone $URL client
  cd client
  git checkout @{u}
  git status

no status is printed, but instead an error message:

  fatal: HEAD does not point to a branch

(This error message when running "git branch" persists even after
checking out other things - it only stops after checking out a branch.)

This is because "git status" reads the reflog when determining the "HEAD
detached" message, and thus attempts to DWIM "@{u}", but that doesn't
work because HEAD no longer points to a branch.

Therefore, when calculating the status of a worktree, tolerate dangling
marks. This is done by adding an additional parameter to
dwim_ref() and repo_dwim_ref().

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -151,12 +151,14 @@ struct argv_array;
 void expand_ref_prefix(struct argv_array *prefixes, const char *prefix);
 
 int expand_ref(struct repository *r, const char *str, int len, struct object_id *oid, char **ref);
-int repo_dwim_ref(struct repository *r, const char *str, int len, struct object_id *oid, char **ref);
+int repo_dwim_ref(struct repository *r, const char *str, int len,
+		  struct object_id *oid, char **ref, int nonfatal_dangling_mark);
 int repo_dwim_log(struct repository *r, const char *str, int len, struct object_id *oid, char **ref);
 static inline int dwim_ref(const char *str, int len, struct object_id *oid,
-			   char **ref)
+			   char **ref, int nonfatal_dangling_mark)
 {
-	return repo_dwim_ref(the_repository, str, len, oid, ref);
+	return repo_dwim_ref(the_repository, str, len, oid, ref,
+			     nonfatal_dangling_mark);
 }
 int dwim_log(const char *str, int len, struct object_id *oid, char **ref);