scripts/git: Quote shell arguments where possible

Most shell arguments should be quoted to avoid mistakes.

But since all branch names are hard-coded, or supplied by the script user,
we don't need to be too concerned about command injection.

Quoting all shell arguments would take a major refactor.
(Probably using arrays.)

Part of 31314.

scripts/git/git-push-all.sh

@@ -171,7 +171,7 @@ fi
 if [ "$PUSH_SAME" -eq 0 -a "$TEST_BRANCH_PREFIX" ]; then
   NEW_PUSH_BRANCHES=
   for b in $PUSH_BRANCHES; do
-    PUSH_COMMIT=`git rev-parse $b`
+    PUSH_COMMIT=`git rev-parse "$b"`
     SKIP_UPSTREAM=
     for u in $DEFAULT_UPSTREAM_BRANCHES $UPSTREAM_BRANCHES; do
       UPSTREAM_COMMIT=`git rev-parse "$u"`