narrow: remove assertion about working copy being clean
The user can always modify the working copy, including while they're
running `hg tracked --remove-include <path>`. Nothing really bad
happens when they do that, and we already have code for printing a
nice warning, so we can safely remove the assertion we had.
Differential Revision: https://phab.mercurial-scm.org/D10063
$ 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