contrib: make simplemerge script pass context-like objects
`simplemerge()` will soon require context-like objects to work. Create a simple
context-like object that wraps the requested files and can be passed to the new
API.
Differential Revision: https://phab.mercurial-scm.org/D378
test-ancestor.py
test-backwards-remove.t
test-branch-tag-confict.t
test-casecollision.t
test-check-commit.t
test-check-execute.t
test-check-pyflakes.t
test-check-pylint.t
test-check-shbang.t
test-contrib-check-code.t
test-contrib-check-commit.t
test-diff-issue2761.t
test-diff-newlines.t
test-diff-reverse.t
test-diff-subdir.t
test-dirstate-nonnormalset.t
test-doctest.py
test-duplicateoptions.py
test-empty-dir.t
test-empty.t
test-excessive-merge.t
test-hghave.t
test-imports-checker.t
test-issue1089.t
test-issue1993.t
test-issue842.t
test-locate.t
test-lrucachedict.py
test-manifest.py
test-match.py
test-merge-default.t
test-merge2.t
test-merge5.t
test-push-checkheads-pruned-B1.t
test-push-checkheads-pruned-B6.t
test-push-checkheads-pruned-B7.t
test-push-checkheads-superceed-A1.t
test-push-checkheads-superceed-A4.t
test-push-checkheads-superceed-A5.t
test-push-checkheads-superceed-A8.t
test-push-checkheads-unpushed-D1.t
test-push-checkheads-unpushed-D6.t
test-push-checkheads-unpushed-D7.t
test-rename.t
test-revlog-packentry.t
test-run-tests.py
test-terse-status.t
test-unified-test.t
test-update-dest.t
test-update-reverse.t
test-xdg.t