ui: option to preserve the progress bar

Some interface like the fast-import format of git are meant for pipe
processing. It can be still desirable to have progress bars in this
case, but clearing the bar after each output operation is not desirable.

Differential Revision: https://phab.mercurial-scm.org/D6843

mercurial/ui.py

@@ -1072,6 +1072,13 b' class ui(object):'
         Label names take the form of "topic.type". For example, ui.debug()
         issues a label of "ui.debug".
 
+        Progress reports via stderr are normally cleared before writing as
+        stdout and stderr go to the same terminal. This can be skipped with
+        the optional keyword argument "keepprogressbar". The progress bar
+        will continue to occupy a partial line on stderr in that case.
+        This functionality is intended when Mercurial acts as data source
+        in a pipe.
+
         When labeling output for a specific command, a label of
         "cmdname.type" is recommended. For example, status issues
         a label of "status.modified" for modified files.
@@ -1087,7 +1094,8 b' class ui(object):'
                 self._buffers[-1].extend(args)
             return
 
-        # inliend _writenobuf() for speed
+        # inlined _writenobuf() for speed
+        if not opts.get(r'keepprogressbar', False):
         self._progclear()
         msg = b''.join(args)
 
@@ -1126,6 +1134,7 b' class ui(object):'
 
     def _writenobuf(self, dest, *args, **opts):
         # update write() as well if you touch this code
+        if not opts.get(r'keepprogressbar', False):
         self._progclear()
         msg = b''.join(args)