bash-completion: Fix __git_ps1 to work with "set -u"

Define several variables in __git_ps1 to avoid errors under "set -u" semantics.

__git_ps1 seems to have been missed when the rest of the file was fixed in
25a31f8.

Signed-off-by: Andrew Sayers <andrew-git@pileofstuff.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -84,8 +84,8 @@ __git_ps1 ()
 {
 	local g="$(__gitdir)"
 	if [ -n "$g" ]; then
-		local r
-		local b
+		local r=""
+		local b=""
 		if [ -f "$g/rebase-merge/interactive" ]; then
 			r="|REBASE-i"
 			b="$(cat "$g/rebase-merge/head-name")"
@@ -127,11 +127,11 @@ __git_ps1 ()
 			}
 		fi
 
-		local w
-		local i
-		local s
-		local u
-		local c
+		local w=""
+		local i=""
+		local s=""
+		local u=""
+		local c=""
 
 		if [ "true" = "$(git rev-parse --is-inside-git-dir 2>/dev/null)" ]; then
 			if [ "true" = "$(git rev-parse --is-bare-repository 2>/dev/null)" ]; then