chore: fix TestMirrorPull on older git (2.34.1) installation (#12134)

`TestMirrorPull` is currently failing when run on git 2.34.1 in the `testing-integration.yml` workflow: https://codeberg.org/forgejo-integration/forgejo/actions/runs/16661/jobs/1/attempt/1#jobstep-5-2539  Began to fail after #11909 when additional checks on pull mirror configuration was added.

This PR addresses the issue and has been manually tested against the same git version:
```
$ git --version
git version 2.34.1

$ make test-sqlite#TestMirrorPull 2>&1
...
=== TestMirrorPull/migrate_from_repo_config_credentials (tests/integration/mirror_pull_test.go:238)
PASS
```

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### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
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Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12134
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Beowulf <beowulf@beocode.eu>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>

tests/integration/mirror_pull_test.go

@@ -492,7 +492,7 @@ func waitForPullMirror(t *testing.T, mirrorName, expectedSha string) {
 }
 
 func getGitConfig(t *testing.T, configFile, configPath string) string {
-	stdout, stderr, err := process.GetManager().Exec("getGitConfig", "git", "config", "get", "--file", configFile, configPath)
+	stdout, stderr, err := process.GetManager().Exec("getGitConfig", "git", "config", "--get", "--file", configFile, configPath)
 	require.NoError(t, err, "fetch config %s failed: git stderr: %s", configPath, stderr)
 	return strings.TrimSpace(stdout)
 }