test-merge-tool: Make sure no hgmerge can be found in $PATH
The merge tool selecting algorithm is hardcoded to look for hgmerge in $PATH
before it falls back to use internal merge. This fixes the test for this
fallback to be tolerant to existing hgmerges.
adding a
abort: username 'foo\nbar1' contains a newline
abort: username 'foo\nbar2' contains a newline
transaction abort!
rollback completed
abort: username 'foo\nbar3' contains a newline!