run-tests.py: pass -L/--label to simplemerge
This should give us conflict markers that don't change from one run
to another.
Remove some sed's/egrep's from some tests.
Is there an error message when trying to diff non-existing files?
found: No such file or directory
not: No such file or directory
Is there an error message when trying to add non-existing files?
found: No such file or directory
not: No such file or directory