merge: use operation-provided labels (ex: dest/source) in several merge-tools Tools that did not use labels already, used only one, or used some label other than the exact strings of "local" or "other" were unmodified. Cases that used the label "base" were modified as well, if they were otherwise changed in this CL; "merged" was *not* changed. There are other possible changes we might want to make, but I didn't: - bcompare (linux and osx) uses the labels "parent1" and "parent2" instead of "local" and "other", so it was left alone, even though beyondcompare3 (windows) *was* changed. - araxis used the labels "Other", "Base", and "Local :$local", so it was also left alone. - UltraCompare didn't provide a label for 'local', just for 'base' and 'other', so it was left alone. Differential Revision: https://phab.mercurial-scm.org/D2012