tests/test-issue619

run-tests: detect when hghave fails to check for a feature and fail test

hghave exitcode != 0 might mean that hghave failed to check for feature
avaibility. Detect those cases, and fail the test, instead of skipping it.

#!/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