run-tests: fix _checkhglib() so it's correct when using --with-hg.
Since this makes the warning less likely, upgrade it to a "real"
warning to stderr.
#!/bin/sh
mkdir t
cd t
hg init
echo a > a
hg ci -Ama -d '1000000000 0'
echo b > b
hg branch b
hg ci -Amb -d '1000000000 0'
hg co -C 0
echo fast-forward
hg merge b
hg ci -Ammerge -d '1000000000 0'
echo bogus fast-forward should fail
hg merge b
echo done