tests: add explicit coverage for update_hash_refs from rewriteutil
I couldn't find any evidence this is covered by tests in core (but there's a
good chance I missed it). We've seen a cute bug in that code, but first
let's just cover the cases that work correctly.
Differential Revision: https://phab.mercurial-scm.org/D11231
$ hg init
$ touch a
$ unset HGUSER
$ echo "[ui]" >> .hg/hgrc
$ echo "username= foo" >> .hg/hgrc
$ echo " bar1" >> .hg/hgrc
$ hg ci -Am m
adding a
abort: username 'foo\nbar1' contains a newline
[255]
$ rm .hg/hgrc
$ HGUSER=`(echo foo; echo bar2)` hg ci -Am m
adding a
abort: username 'foo\nbar2' contains a newline
[255]
$ hg ci -Am m -u "`(echo foo; echo bar3)`"
adding a
transaction abort!
rollback completed
abort: username 'foo\nbar3' contains a newline
[50]