simplemerge: add optional context parameters to simplemerge Rename the existing parameters for clarity. These will, in subsequent patches, allow callers to redirect reads (of the three sides of the merge) and writes (of the result) to the given contexts, instead of using the filesystem. While in most cases, the writes will go to a workingfilectx, this opens the door for it to be a memfilectx in the case of an in-memory merge. Repo will be necessary in a subsequent comit. Differential Revision: https://phab.mercurial-scm.org/D372

simplemerge: extract verifytext as a helper function This will be used in a subsequent commit. Differential Revision: https://phab.mercurial-scm.org/D371

commit: use context manager with dirstateguard When I wrote 5ac845ca059a (commit: don't let failed commit with --addremove update dirstate (issue5645), 2017-07-31), Durham's 609606d21765 (rebase: use one dirstateguard for when using rebase.singletransaction, 2017-07-20) had not yet landed, so I had to write it in the old-fashioned way. Now that Durham's patch is in, we can simplify by using a context manager. Differential Revision: https://phab.mercurial-scm.org/D406

commit: move dirstateguard creation out of try-block This is just a simple refactoring to make the next patch simpler. If the dirstateguard constructor raises an exception, the finally-block won't do anything anyway, so this is functionally equivalent (and there is no except-block). Differential Revision: https://phab.mercurial-scm.org/D405