config-schema: fix default for git.write-change-id-header

#6475 changed the default value for `git.write-change-id-header` to
`true` but this was not mirrored in the schema's default, causing the
new `test_config_get_yields_values_consistent_with_schema_defaults` to
fail.

Author: jgreitemann

cli/src/config-schema.json

@@ -468,7 +468,7 @@
                 "write-change-id-header": {
                     "type": "boolean",
                     "description": "Whether the change id should be stored in the Git commit object",
-                    "default": false
+                    "default": true
                 },
                 "executable-path": {
                     "type": "string",