Fix another "==" -> "=" bashism.

I notice it doesn't quite work on my Ubuntu system - if I "cd version"
and run "../redo", I get the following output:

   redo  all
   redo    vars
   redo      gitvars
   [: 24: %H: unexpected operator
   [: 24: %d: unexpected operator
   [: 24: %ci: unexpected operator

...and then a hang.

version/gitvars.do

@@ -13,7 +13,7 @@ while read line; do
 	if [ "$x" != "$line" ]; then
 		# git didn't substitute it
 		x=${x%\$}  # remove trailing $
-		if [ "$x" == "%d" ]; then
+		if [ "$x" = "%d" ]; then
 			tag=$(git describe --match="$PROD-*")
 			x="(tag: $tag)"
 		else