completions/git: add more special refs

These are things like .git/HEAD, i.e. the ones that are typically not
namespaced under .git/refs.  The list is taken from gitrevisions(7).

share/completions/git.fish

@@ -82,7 +82,8 @@ end
 function __fish_git_heads
     set -l gitdir (__fish_git rev-parse --git-dir 2>/dev/null)
     or return # No git dir, no need to even test.
-    for head in HEAD FETCH_HEAD ORIG_HEAD MERGE_HEAD
+    for head in HEAD FETCH_HEAD ORIG_HEAD MERGE_HEAD REBASE_HEAD REVERT_HEAD \
+        CHERRY_PICK_HEAD BISECT_HEAD AUTO_MERGE
         if test -f $gitdir/$head
             echo $head
         end