Issue a warning if "-r ." is used with two working directory parents. Rationale for not aborting instead: The first parent is usually more important as it is the local branch during a merge and commands like 'hg diff' and 'hg diff -r.' behave still identically (except for the warning of course). Added a test for log -r. with one and two parents.

uisetup called
ui.parentui is None
reposetup called for a
ui == repo.ui
Foo
uisetup called
ui.parentui is None
reposetup called for a
ui == repo.ui
reposetup called for b
ui == repo.ui
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
uisetup called
ui.parentui is None
Bar