merge: set custom conflict label for base commit Differential Revision: https://phab.mercurial-scm.org/D12021

rebase: set custom conflict label for base commit Differential Revision: https://phab.mercurial-scm.org/D12020

simplemerge: take over formatting of label from `filemerge` The padding we do of conflict labels depends on which conflict marker style is used. For two-way conflict markers (the default), the length of the base label shouldn't matter. It does before this patch, however. This patch moves the formatting from `filemerge` to `simplemerge`. The latter knows which conflict marker style to use, so it can easily decide about the padding. This change will allow us to use more descriptive "base" labels without causing illogical padding in 2-way markers. I'll do that next. One wrinkle is that we pass the same labels to external merge tools. I decided to change that in this patch to be simpler: no padding, and no ellipsis to fit within 80 columns. My reasoning is that the typical external, 3-or-4-panel merge tool doesn't show the labels on top of each others, so the padding doesn't make sense there. The ellipsis is probably not necessary because the external tools probably have their own way of dealing with long labels. Also, we limit them to "80 - 8" to fit the "<<<<<<< " before, which is almost definitely not what an external tool would put there. Differential Revision: https://phab.mercurial-scm.org/D12019