completion: Fix GIT_PS1_SHOWDIRTYSTATE to prevent unbound variable errors.

Signed-off-by: NTed Pavlic <ted@tedpavlic.com>
Acked-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -125,7 +125,7 @@ __git_ps1 ()
 		local w
 		local i
 
-		if test -n "$GIT_PS1_SHOWDIRTYSTATE"; then
+		if test -n "${GIT_PS1_SHOWDIRTYSTATE-}"; then
 			if test "$(git config --bool bash.showDirtyState)" != "false"; then
 				git diff --no-ext-diff --ignore-submodules \
 					--quiet --exit-code || w="*"