Fix svn client incorrectly detecting "history scheduled with commit" with changelists

Scenario:
1. Add a new file X to branch BrA, commit
2. svn merge X from branch BrA to branch BrB in working copy (status "A +")
3. Modify file Y in working copy, send ONLY file Y (status "M) to changelist CL
4. rbt post --svn-changelist=CL

Expected: post succeeds, because changelist only contains modified files
Actual: post fails with "Please try again with
'--svn-show-copies-as-adds=y/n'"; this is because rbt "sees" file X, despite
explicitly only posting Y for review.

This patch modifies history_scheduled_with_commit in rbtools/clients/svn.py to
pass the --changelist option, if present, down to 'svn st'

Testing done:
* The scenario described above now works as expected.
* Unit tests pass.

Fixes bug 3248.