Stop using the "x$FOO" idiom in git-resquash.sh

The new version of shellcheck says that that this idiom is
unnecessary, and its presence here is making the shellcheck tests
fail.

No changes file needed, since this is not user-facing code.

scripts/git/git-resquash.sh

@@ -7,7 +7,7 @@ set -e
 
 PARENT="$1"
 
-if test "x$PARENT" = "x"; then
+if test "$PARENT" = ""; then
     echo "You must specify the parent branch."
     exit 1
 fi
@@ -32,12 +32,12 @@ else
     USE_KEEP_BASE=1
 fi
 
-if test "x$USE_KEEP_BASE" = "x1" ; then
+if test "$USE_KEEP_BASE" = "1" ; then
     exec git rebase -i --autosquash --keep-base "${PARENT}"
 else
     REV=$(git log --reverse --format='%H' "${PARENT}..HEAD" | head -1)
 
-    if test "x${REV}" = "x"; then
+    if test "${REV}" = ""; then
         echo "No changes here since ${PARENT}"
         exit 1
     fi