bug#14459: git vc-annotate fails in detached HEAD state

[Claudio Bley]

Hi.

1. I open a git versioned file with vc-git enabled in a directory
   currently in detached HEAD state (e.g. after checking out a tag)
2. notice the mode line says "Git:!"

Now, when I do

C-x v g (vc-annotate)

The annotation buffer will be displayed, only saying

,----
| fatal: bad revision ''
`----

Apparently, vc-git-working-revision calls "git symbolic-ref HEAD",
which - in this case - gives:

$ git symbolic-ref HEAD
fatal: ref HEAD is not a symbolic ref

So, vc-git-working-revision returns an empty string as the current
"revision" vc-mode refers to.

M-: (vc-git-working-revision (buffer-file-name))

-> ""

When calling vc-annotate, the vc-git-annotate-command gets called with
the empty string for the rev parameter leading to the error.

This patch fixes it for me:

----------------- 8< --------------------- 8< -------------
diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index 06474cb..63a7115 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -885,7 +885,7 @@ or BRANCH^ (where \"^\" can be repeated)."
 
 (defun vc-git-annotate-command (file buf &optional rev)
   (let ((name (file-relative-name file)))
-    (vc-git-command buf 'async nil "blame" "--date=iso" "-C" "-C" rev "--" 
name)))
+    (vc-git-command buf 'async nil "blame" "--date=iso" "-C" "-C" (if (> 
(length rev) 0) rev) "--" name)))
 
 (declare-function vc-annotate-convert-time "vc-annotate" (time))
 
-- 
Claudio