mergestate: use an early return for trivial merges `filemerge.filemerge()` returns `None` if no merge was needed because the two sides were identical. I'd like to move that to the caller. This is a little refactoring to prepare for that. Differential Revision: https://phab.mercurial-scm.org/D12153

simplemerge: remove now-unused arguments The `ui` argument is no longer needed now that we don't handle `--print` in the library code. Differential Revision: https://phab.mercurial-scm.org/D12151

merge: fix documented order of `labels` argument Differential Revision: https://phab.mercurial-scm.org/D12150

filemerge: when not keeping premerge, don't write markers to context When premerge is enabled (as it is for non-binary inputs by default) and the markers are not kept, we currently still write it to the output context and then restore the previous content right after. With the refactoring in the previous patch, we can easily avoid that step and instead write the output in the opposite case (i.e. when it's successful or when the markers are supposed to be kept). Differential Revision: https://phab.mercurial-scm.org/D12149

simplemerge: move printing of merge result to extension The `mercurial.simplemerge` module still has some command-lines processing that doesn't belong in such a low-level module. This patch moves the handling of `hg simplemerge --print` to the extension by having `mercurial.simplemerge.simplemerge()` return the merged text. Differential Revision: https://phab.mercurial-scm.org/D12148

simplemerge: remove code for checking binary input now that callers do it The callers now do the checking for binary inputs and handle warnings and/or errors, so we can remove that code from the low-level `simplemerge` module now. After this patch we just raise an error unless the caller told us to allow binary inputs. Differential Revision: https://phab.mercurial-scm.org/D12169

simplemerge: let filemerge check for binary inputs This is similar to the previous patch, but here we put a specialized copy of `simplemerge._verifytext()` in the the `filemerge` module instead. Differential Revision: https://phab.mercurial-scm.org/D12147

simplemerge: let extension check for binary inputs (unless `--text`) The `simplemerge` module does too much UI-related things. This patch puts a specialized copy of `simplemerge._verifytext()` in the simplemerge extension. Differential Revision: https://phab.mercurial-scm.org/D12146

simplemerge: store input data in MergeInput I want to be able to update the file contents for support for partial conflict resolution. Having the text available (and later settable) on `MergeInput` will enable that. Differential Revision: https://phab.mercurial-scm.org/D12144

simplemerge: convert MergeInput to regular, non-attr.ib class I'm about to add more logic and state to it. Differential Revision: https://phab.mercurial-scm.org/D12145