view CONTRIBUTING @ 47437:7a430116f639

ui: add a context manager for silencing the ui (pushbuffer+popbuffer)

We often silence the ui by calling `ui.pushbuffer()` followed (a later
in the code) by `ui.popbuffer()`. These places can be identified by
the fact that they ignore the output returned from
`ui.popbuffer()`. Let's create a context manager for these cases, to
avoid repetition, and to avoid accidentally leaving the ui silent on
exceptions. I deliberately called the new function `silent()` instead
of `buffered()`, because it's just an implementation detail that it
uses `pushbuffer()` and `popbuffer()`. We could later optimize it to
not buffer the output.

Differential Revision: https://phab.mercurial-scm.org/D10884

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