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

mercurial/ui.py

@@ -1481,7 +1481,7 class ui(object):
 
     def _progclear(self):
         """clear progress bar output if any. use it before any output"""
-        if '_progbar' not in vars(self): # nothing loaded yet
+        if not haveprogbar(): # nothing loaded yet
             return
         if self._progbar is not None and self._progbar.printed:
             self._progbar.clear()
@@ -1785,3 +1785,6 def getprogbar(ui):
         # this is how the extension used to work but feel free to rework it.
         _progresssingleton = progress.progbar(ui)
     return _progresssingleton
+
+def haveprogbar():
+    return _progresssingleton is not None

tests/test-subrepo-deep-nested-change.t

@@ -63,6 +63,14 Preparing the 'main' repo which depends
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   updating [===========================================>] 3/3\r (no-eol) (esc)
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   updating [===========================================>] 1/1\r (no-eol) (esc)
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   updating to branch default
@@ -154,7 +162,25 Clone main
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   updating [===========================================>] 1/1\r (no-eol) (esc)
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   updating to branch default