tests/test-backwards-remove.t

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.

  $ hg init repo
  $ cd repo
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls -A
  .hg
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls -A
  .hg
  a