transaction: fix __repr__() and make the default name bytes
This likely was always wrong on py3, since going back to
aff5996f3043, these
were added as a r-strings. Callers seem to always be supplying bytes, which
makes the `b'/'.join(...)` part OK, but then bytes can't be interpolated into
str with "%s", so it wouldn't have worked in either case.
$ hg init repo
$ cd repo
$ "$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