ui.write: don't clear progress bar when writing to a buffer

ui.write: don't clear progress bar when writing to a buffer

ui.write() has 2 modes: buffered and unbuffered. In buffered mode, we
capture output before writing it. This is how changeset printing works,
for example.

Previously, we were potentially clearing the progress bar for every
call to ui.write(). In buffered mode, this clearing was useless because
the clearing function would be called again before actually writing
the buffered data.

This patch stops the useless calling of _progclear() unless we are
actually writing data. During changeset printing with the default
template, this removes ~6 function calls per changeset, making
changeset printing slightly faster.

before: 23.76s
after:  23.35s
delta:  -0.41s (98.3% of original)

--- a/mercurial/ui.py	Sun Nov 22 21:20:08 2015 -0500
+++ b/mercurial/ui.py	Sat Nov 14 17:14:14 2015 -0800
@@ -612,10 +612,10 @@
         "cmdname.type" is recommended. For example, status issues
         a label of "status.modified" for modified files.
         '''
-        self._progclear()
         if self._buffers:
             self._buffers[-1].extend([str(a) for a in args])
         else:
+            self._progclear()
             for a in args:
                 self.fout.write(str(a))