procutil: use unbuffered stdout on Windows

Windows doesn't support line buffering, treating it as fully buffered. This
causes output of slow commands to stutter. We use unbuffered instead.

mercurial/utils/procutil.py

@@ -42,8 +42,12 b' def isatty(fp):'
 
 # glibc determines buffering on first write to stdout - if we replace a TTY
 # destined stdout with a pipe destined stdout (e.g. pager), we want line
-# buffering
+# buffering (or unbuffered, on Windows)
 if isatty(stdout):
+    if pycompat.iswindows:
+        # Windows doesn't support line buffering
+        stdout = os.fdopen(stdout.fileno(), r'wb', 0)
+    else:
     stdout = os.fdopen(stdout.fileno(), r'wb', 1)
 
 if pycompat.iswindows: