formatter: fix handling of None value in templater mapping
For historical reasons, None in mapping dict means there's no such keyword,
and falls back to b"". That's fine in log templates where mapping item is
generally a callable returning a value (which may be None,) but the formatter
directly puts an "evaluated" value in the mapping. So the None value has
to be lifted to wrappedvalue(None) to avoid confusion in the template engine.
$ hg init
$ "$PYTHON" -c 'open("a", "wb").write(b"confuse str.splitlines\nembedded\rnewline\n")'
$ hg ci -Ama -d '1 0'
adding a
$ echo clean diff >> a
$ hg ci -mb -d '2 0'
$ hg diff -r0 -r1
diff -r 107ba6f817b5 -r 310ce7989cdc a
--- a/a Thu Jan 01 00:00:01 1970 +0000
+++ b/a Thu Jan 01 00:00:02 1970 +0000
@@ -1,2 +1,3 @@
confuse str.splitlines
embedded\r (no-eol) (esc)
newline
+clean diff