test: explicitly "add" file before some commit in test-filecache.py

`hg commit -A` will revert the `hg addremove` step if the commit fails. However
`hg rollback` currently does not.

We are about to improve internal consistency around transaction and dirstate and the behavior of `hg rollback` will align on the other behavior in the process.

Before doing so, we make sure the test is using a separate call to `hg add` to
avoid the test scenario to be affected by that future change.

note: the behavior change for `hg rollback` seems fine as it affect a niche
usecase and `hg rollback` usage have been strongly discouraged for a while.

tests/test-filecache.py

@@ -165,7 +165,7 b' def fakeuncacheable():'
 
 def test_filecache_synced():
     # test old behavior that caused filecached properties to go out of sync
-    os.system('hg init && echo a >> a && hg ci -qAm.')
+    os.system('hg init && echo a >> a && hg add a && hg ci -qm.')
     repo = hg.repository(uimod.ui.load())
     # first rollback clears the filecache, but changelog to stays in __dict__
     repo.rollback()