ui: add config knob to redirect status messages to stderr (API) This option can be used to isolate structured output from status messages. For now, "stdio" (stdout/err pair) and "stderr" are supported. In future patches, I'll add the "channel" option which will send status messages to a separate command-server channel with some metadata attached, maybe in CBOR encoding. This is a part of the generic templating plan: https://www.mercurial-scm.org/wiki/GenericTemplatingPlan#Sanity_check_output .. api:: Status messages may be sent to a dedicated stream depending on configuration. Don't use ``ui.status()``, etc. as a shorthand for conditional writes. Use ``ui.write()`` for data output.

--- a/mercurial/configitems.py	Sat Nov 03 20:53:31 2018 +0900
+++ b/mercurial/configitems.py	Sat Nov 03 19:42:50 2018 +0900
@@ -1181,6 +1181,9 @@
             '{ifeq(branch, "default", "", "{branch} ")}'
             '- {author|user}: {desc|firstline}')
 )
+coreconfigitem('ui', 'message-output',
+    default='stdio',
+)
 coreconfigitem('ui', 'nontty',
     default=False,
 )