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rhg: add ui.plain() and check it before showing relative paths in status

Adds a very basic replica of `ui.plain()`.

Differential Revision: https://phab.mercurial-scm.org/D10912
17 jobs for topic/default/rhg-review-ready in 49 minutes and 17 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Tests
passed check-pytype-py3 #251343

00:27:11

passed checks-py2 #251330

00:05:28

passed checks-py3 #251331

00:05:01

passed rust-cargo-test-py2 #251332

00:04:17

passed rust-cargo-test-py3 #251333

00:04:53

passed test-py2 #251334

00:19:25

passed test-py2-chg #251341

00:48:39

passed test-py2-pure #251336

00:28:43

passed test-py2-rust #251338

00:29:05

passed test-py3 #251335

00:15:36

passed test-py3-chg #251342

00:14:01

passed test-py3-pure #251337

00:23:23

passed test-py3-rhg #251340

00:18:13

passed test-py3-rust #251339

00:31:56

manual windows-py3 #251344
windows allowed to fail manual
manual windows-py3-pyox #251345
windows allowed to fail manual
 
  Phabricator
passed phabricator-refresh #251346

00:00:31