completion: simplify "current branch" in __git_ps1()

As I very often work on a detached HEAD, I found it pretty confusing
when __git_ps1() said 'some-name'.  Did I create a branch with that name
by mistake, or do I happen to be on a commit with that exact tag?

This patch fixes the issue by enclosing non branch names in a pair of
parentheses when used to substitute %s token in __git_ps1() argument.

It also fixes a small bug where the branch part is left empty when
.git/HEAD is unreadable for whatever reason.  The output now says
"(unknown)".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -106,13 +106,14 @@ __git_ps1 ()
 			if [ -f "$g/BISECT_LOG" ]; then
 				r="|BISECTING"
 			fi
-			if ! b="$(git symbolic-ref HEAD 2>/dev/null)"; then
-				if ! b="$(git describe --exact-match HEAD 2>/dev/null)"; then
-					if [ -r "$g/HEAD" ]; then
-						b="$(cut -c1-7 "$g/HEAD")..."
-					fi
-				fi
-			fi
+
+			b="$(git symbolic-ref HEAD 2>/dev/null)" || {
+				b="$(git describe --exact-match HEAD 2>/dev/null)" ||
+				b="$(cut -c1-7 "$g/HEAD" 2>/dev/null)..." ||
+				b="unknown"
+
+				b="($b)"
+			}
 		fi
 
 		local w