histedit: fix rollup prompting for a commit message (issue4606) Commit 090da03361c5 broke histedit's rollup by causing it to open the editor. Turns out I missed a spot where the rollup option was read. This fixes that and adjusts the test to catch this case.

test-hook.t: don't directly use redirect to /dev/null in hook for Windows This goes with 57f1dbc99631. External hooks are run in cmd.exe, which doesn't know about /dev/null, but sh can handle it.