fuzz: compile xdiff.cc with -std=c++17
Otherwise the build would fail as follows:
In file included from xdiff.cc:13:
./fuzzutil.h:23:10: fatal error: 'third_party/absl/types/optional.h' file not
found
#include "third_party/absl/types/optional.h"
$ 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!
[255]