Merge branch 'rr/prompt-revert-head'

The prompt string generator did not notice when we are in a middle
of a "git revert" session.

* rr/prompt-revert-head:
  bash: teach __git_ps1 about REVERT_HEAD

contrib/completion/git-prompt.sh

@@ -282,6 +282,8 @@ __git_ps1 ()
 				r="|MERGING"
 			elif [ -f "$g/CHERRY_PICK_HEAD" ]; then
 				r="|CHERRY-PICKING"
+			elif [ -f "$g/REVERT_HEAD" ]; then
+				r="|REVERTING"
 			elif [ -f "$g/BISECT_LOG" ]; then
 				r="|BISECTING"
 			fi