test: explicitly "add" file before some commit in test-rollback.t

`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-rollback.t

@@ -2,8 +2,8 b' setup repo'
   $ hg init t
   $ cd t
   $ echo a > a
-  $ hg commit -Am'add a'
-  adding a
+  $ hg add a
+  $ hg commit -m 'add a'
   $ hg verify -q
   $ hg parents
   changeset:   0:1f0dee641bb7