[open-] fix open-file for - in cmdlogs #2582

[midichef]

Closes #2582 and #2583, which were bugs I introduced in #2442. The trouble is that the wrong file is tested for being a terminal. When command | vd is run in a terminal, duptty() eventually turns sys.stdin into a terminal. The right file to test is stdinSource.fptext.

@anjakefala How can I add some tests that run commands in a shell? I'd like to add tests that pipe to vd, like:
cat sample.tsv | vd -
cat sample.tsv | vd -p -
cat sample.tsv | vd -p open-file.vdj

They'll be good for testing the line of code added in this commit, when I submit a patch to make piped data be binary, instead of text.

[midichef]

vd -p - would lead to a nonresponsive terminal. Similar to the way that vd - used to, before 49a1238. So I've patched that as well.