Filter out HEAD refs from magit-name-rev.

HEAD refs are nasty because they're highly contingent.
Branches, tags, and remotes all require some commit to be made
in order for the ref to point to a different rev,
but HEADs just need something else to be checked out.

Also, filtering out HEADs makes the name more portable.
This is useful for Magithub, where the name may be passed on to GitHub.

magit.el

@@ -521,9 +521,23 @@ return nil."
 (defun magit-name-rev (rev)
   "Return a human-readable name for REV.
 Unlike git name-rev, this will remove tags/ and remotes/ prefixes
-if that can be done unambiguously."
+if that can be done unambiguously.  In addition, it will filter
+out revs involving HEAD."
   (when rev
     (let ((name (magit-git-string "name-rev" "--no-undefined" "--name-only" rev)))
+      ;; There doesn't seem to be a way of filtering HEAD out from name-rev,
+      ;; so we have to do it manually.
+      ;; HEAD-based names are too transient to allow.
+      (when (string-match "^\\(.*\\<HEAD\\)\\([~^].*\\|$\\)" name)
+        (let ((head-ref (match-string 1 name))
+              (modifier (match-string 2 name)))
+          ;; Sometimes when name-rev gives a HEAD-based name,
+          ;; rev-parse will give an actual branch or remote name.
+          (setq name (concat (magit-git-string "rev-parse" "--abbrev-ref" head-ref)
+                             modifier))
+          ;; If rev-parse doesn't give us what we want, just use the SHA.
+          (when (or (null name) (string-match-p "\\<HEAD\\>" name))
+            (setq name (magit-rev-parse ref)))))
       (setq rev (or name rev))
       (when (string-match "^\\(?:tags\\|remotes\\)/\\(.*\\)" rev)
         (let ((plain-name (match-string 1 rev)))