Make actual HEAD commit message optional in the syntax

Since we now render lazy and (in some way) async, we need to
reduce visual flicker. By making the actual commit message
optional, we ensure that the literal `HEAD:` is always styled
correctly.

syntax/dashboard.sublime-syntax

@@ -35,7 +35,7 @@ contexts:
           comment: ahead or behind
           scope: keyword.other.git-savvy.summary-header.ahead-behind
 
-        - match: ^  (HEAD:)\s+([0-9a-f]{7,40}) .+
+        - match: ^  (HEAD:)(?:\s+([0-9a-f]{7,40}) .+)?
           comment: head summary
           scope: comment.git-savvy.summary-header.head-summary
           captures: