git-prompt: change == to = for zsh's sake

When using git-prompt.sh with zsh, __git_ps1 currently errs
when inside a repo with:

__git_ps1:96: = not found

Avoid using non-portable "==" that is only understood by bash
and not zsh. Change to "=" so that the prompt script becomes
usable with zsh again.

Signed-off-by: David J. Malan <malan@harvard.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: dmalan

contrib/completion/git-prompt.sh

@@ -433,7 +433,7 @@ __git_ps1 ()
 	local sparse=""
 	if [ -z "${GIT_PS1_COMPRESSSPARSESTATE}" ] &&
 	   [ -z "${GIT_PS1_OMITSPARSESTATE}" ] &&
-	   [ "$(git config --bool core.sparseCheckout)" == "true" ]; then
+	   [ "$(git config --bool core.sparseCheckout)" = "true" ]; then
 		sparse="|SPARSE"
 	fi
 
@@ -542,7 +542,7 @@ __git_ps1 ()
 		fi
 
 		if [ -n "${GIT_PS1_COMPRESSSPARSESTATE}" ] &&
-		   [ "$(git config --bool core.sparseCheckout)" == "true" ]; then
+		   [ "$(git config --bool core.sparseCheckout)" = "true" ]; then
 			h="?"
 		fi