progress: display progress bar when HGPLAINEXCEPT contains "progress"

This patch changes "progress.shouldprint()" so a feature name is provided to
"ui.plain()" to determine if there is an exception specificed in HGPLAINEXCEPT
for the progress extension.

This will allow user-facing scripts to provide progress output while HGPLAIN
is enabled.

mercurial/help/environment.txt

@@ -69,6 +69,8 b' HGPLAINEXCEPT'
         Preserve internationalization.
     ``revsetalias``
         Don't remove revset aliases.
+    ``progress``
+        Don't hide progress output.
 
     Setting HGPLAINEXCEPT to anything (even an empty string) will
     enable plain mode.

mercurial/progress.py

@@ -18,7 +18,7 b' def spacejoin(*args):'
     return ' '.join(s for s in args if s)
 
 def shouldprint(ui):
-    return not (ui.quiet or ui.plain()) and (
+    return not (ui.quiet or ui.plain('progress')) and (
         ui._isatty(sys.stderr) or ui.configbool('progress', 'assume-tty'))
 
 def fmtremaining(seconds):

tests/test-progress.t

@@ -79,6 +79,12 b' test with delay=0, refresh=0'
 no progress with --quiet
   $ hg -y loop 3 --quiet
 
+test plain mode exception
+  $ HGPLAINEXCEPT=progress hg -y loop 1
+  \r (no-eol) (esc)
+  loop [                                                ] 0/1\r (no-eol) (esc)
+                                                              \r (no-eol) (esc)
+
 test nested short-lived topics (which shouldn't display with nestdelay):
 
   $ hg -y loop 3 --nested