phabricator: add debug logging to show previous node values in `phabsend`
This isn't real useful here, but was very useful showing how `phabsend --fold`
handles commits created by `hg fold` and `hg split`. It introduces a new debug
function and flag instead of using `ui.debug()`, because `--debug` prints out
all of the API chatter.
Differential Revision: https://phab.mercurial-scm.org/D8389
$ 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