filemerge: work with `simplemerge.MergeInput` in `filemerge()`
We currently pass around pairs of file context objects and labels
between functions in the `filemerge` module. I plan to pass around
`simplemerge.MergeInput` instead. This patch prepares for that by
using the type internally in `filemerge.filemerge()`.
Differential Revision: https://phab.mercurial-scm.org/D12017
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