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