properly check for tty when in powershell

[vilmibm]

closes #1581

Only under Powershell we were failing to accurately tell if a command was running attached to a tty or not due to our use of colorableOut. This PR tweaks the check to account for this backports a change made in #1552 that cached the isTerminal check prior to creating a colorable, avoiding the bug.

I had to do a gross hack to fix this and am open to suggestion. colorable only builds the colorable.Writer type if built on windows; this made detecting the above scenario very difficult. I had to make a shim type that is conditionally built in order to get this compiling on all of our CI test nodes.

[mislav]

Nice catch! We can't use IsTerminal on a colorable stream; that will always return false even if it would return true for the original stream.

It looks like the release-cmd branch might inadvertently fix this due to the changes to iostreams.System() I've made there: https://github.com/cli/cli/pull/1552/files#diff-9e30abd45545c3e865502cae5bf624d9

There, we first check IsTerminal and only after that we convert the streams to colorable. That approach does not require us to add any build tags. What do you think?

[vilmibm]

I'm totally fine with that solution! I do not like the build tags hack at all.

I for a second thought that the isTerminal check wouldn't work on powershell anyway, but I just tested it and that will work fine.

However, we wanted to release today, right? It seems unwise to rush releases in today and I'd love to not have things so broken on powershell; can you backport the iostreams changes?

[vilmibm]

I'll leave this open as a reminder but we're just going to focus on getting #1552 in as the fix for this issue.

nevermind, I'll backport the fix from 1552