completion: fix PS1 display during a merge on detached HEAD

If your merge stops in a conflict while on a detached HEAD, recent
completion code fails to show anything.  This was because various cases
added to support the operation-in-progress markers (e.g. REBASE, MERGING)
forgot that they need to set the variable "b" to something for the result
they computed to be displayed at all.

Probably not many people make trial merges on a detached HEAD (which is
tremendously useful feature of git, by the way), and that may be why this
was not noticed for a long time.

Acked-By: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -99,10 +99,10 @@ __git_ps1 ()
 		elif [ -d "$g/rebase-merge" ]; then
 			r="|REBASE-m"
 			b="$(cat "$g/rebase-merge/head-name")"
-		elif [ -f "$g/MERGE_HEAD" ]; then
-			r="|MERGING"
-			b="$(git symbolic-ref HEAD 2>/dev/null)"
 		else
+			if [ -f "$g/MERGE_HEAD" ]; then
+				r="|MERGING"
+			fi
 			if [ -f "$g/BISECT_LOG" ]; then
 				r="|BISECTING"
 			fi