CONTRIBUTING

ui: check for progress singleton when clearing progress bar (issue5684)

A combination of wrapping `ui` and progress bars interrupted by exceptions can
lead to the progress bar not being cleared when the exception error is printed.
This results in corrupted-looking output like this:

```
updating [===============================>                        ] 1/2u
nresolved conflicts (see hg resolve, then hg rebase --continue)
```

This is because in `ui._progclear`, we only check the local reference to the
progress bar, not whether or not there is an instance of the singleton.  When a
progress bar is interrupted by an exception, the exception printing in
`scmutil.callcatch` uses the original instance of the `ui` object, not the
wrapped copy that has `_progbar` set.

When consider whether or not to clear the progress bar, check for the existence
of the singleton, rather than just whether or not we have a local reference to
it.

Differential Revision: https://phab.mercurial-scm.org/D743

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