minor symfony#28340 [appveyor] minor CI fix (nicolas-grekas)

This PR was merged into the 2.8 branch.

Discussion
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[appveyor] minor CI fix

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Should fix this message on appveyor:
`fatal: Not a valid object name HEAD^`

Commits
-------

a391331 [appveyor] minor CI fix

Author: nicolas-grekas

.github/build-packages.php

@@ -14,7 +14,7 @@
 $flags = \PHP_VERSION_ID >= 50400 ? JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE : 0;
 
 foreach ($dirs as $k => $dir) {
-    if (!system("git diff --name-only $mergeBase -- $dir", $exitStatus)) {
+    if (!system("git diff --name-only \"$mergeBase\" -- $dir", $exitStatus)) {
         if ($exitStatus) {
             exit($exitStatus);
         }