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

@@ -575,8 +575,9 b' test rollback'
 
   $ echo foo > f1
   $ hg bookmark tmp-rollback
-  $ hg ci -Amr
+  $ hg add .
   adding f1
+  $ hg ci -mr
   $ hg bookmarks
      X2                        1:925d80f479bb
      Y                         2:db815d6d32e6