extensions: add wrappedfunction() context manager Several extensions exist that temporarily want to wrap a function (at least narrowhg, any many of the extensions in hg-experimental). That's why we have the unwrapfunction() that was introduced in 19578bb84731 (extensions: add unwrapfunction to undo wrapfunction, 2016-08-10). This patch adds a simple wrappedfunction() that returns a context manager. Differential Revision: https://phab.mercurial-scm.org/D472

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