update: use context manager for config override (API)
Note that update wasn't resetting the value before, so any extensions
that called commands.update() and relied on ui.forcemerge being set
after it returned would now have to set it themselves.
(There technically a small API change in all of the patches in this
series, I believe: If extensions relied on the methods to *clear*
ui.forcemerge, then they would have to do that themselves now, because
ui.configoverride() actually restores the previous config, it doesn't
just clear it like these functions did before.)
Differential Revision: https://phab.mercurial-scm.org/D3741
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