Fix pre-commit script's use of ==.

pre-commit.posix

@@ -21,14 +21,14 @@ for dir in `find . -maxdepth 1 -type d | grep -v "\.$" | grep -v "\./\." | sed "
             break;
         fi;
 
-        if [ $file == $dir/README.md ]; then
+        if [ $file = $dir/README.md ]; then
             run_validator markdown $dir;
-        elif [ $file == $dir/Metadata.xml ]; then
+        elif [ $file = $dir/Metadata.xml ]; then
             run_validator metadata $dir;
         elif [ `echo $file | grep "^$dir\/Screenshots\/"` ]; then
             run_validator screenshot $dir;
         else
-            if [ $build_done == "false" ]; then
+            if [ $build_done = "false" ]; then
                 run_validator build $dir mdtool-build;
                 build_done="true"
             fi;