tests/test-rollback.out

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.

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1 files, 1 changesets, 1 total revisions
changeset:   0:0acdaf898367
tag:         tip
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     test

rolling back last transaction
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A a