filemerge: always define a "base" label It simplifies the code and improves the conflict markers to always have the "base" label defined. Differential Revision: https://phab.mercurial-scm.org/D12016

filemerge: set default labels a little earlier By setting the default labels a little earlier, we can rely on them always being set, as far as I can tell. It may actually even be fine to rely on that even if we don't set them earlier, but it makes more sense to me to do it. Differential Revision: https://phab.mercurial-scm.org/D12015

filemerge: remove unused arguments from `_merge()` Differential Revision: https://phab.mercurial-scm.org/D12014

simplemerge: take arguments as annotated context objects The labels we put in conflict markers are formatted so the part before the ':' (typically says things like "local") is padded so the ':' is aligned among the labels. That means that if you specify a long label for "base" but the conflict marker style is "merge" (i.e. 2-way), the other two will have unwanted padding. We often don't specify a label for the base, so we don't notice the problem (and it may very well be that it didn't exist before my D11972). I think the best fix is to pass the labels along with the context objects, so the low-level code that switches on the marker style to use (i.e. `simplemerge`) can do the formatting. This patch starts doing that by passing a fully-formatted label to `simplemerge`. A coming patch will move the formatting to `simplemerge`. Differential Revision: https://phab.mercurial-scm.org/D12013

filemerge: make `labels` a required arg for `_premerge()` We always pass labels, and it's always a non-empty list, so we can simplify a bit. Differential Revision: https://phab.mercurial-scm.org/D12012

filemerge: remove unnecessary check for empty string Looking at the statement just before, the string is clearly never empty (or otherwise falsy). Differential Revision: https://phab.mercurial-scm.org/D12011