helptext: add missing newline to Rust helptext This makes it so the list renders properly. Differential Revision: https://phab.mercurial-scm.org/D12007

unamend: error out when running on merge commit It's better to error out than to produce a bad commit. We do that same in `hg uncommit`. I haven't looked into how much work it is to make them work instead. Differential Revision: https://phab.mercurial-scm.org/D12087

tests: demonstrate how `hg unamend` fails on merge commits When `hg unamend` is run on a merge commit, it seems that it loses the changes from the second parent. Differential Revision: https://phab.mercurial-scm.org/D12086

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