aptible ssh: Correct TTY handling

[krallin]

SSH's default behavior is as follows:

• If a TTY is forced, one is allocated.
• If there is no command, then a TTY is allocated.
• If no-TTY is forced, then none is allocated.
• No TTY is allocated if stdin isn't a TTY.

Unfortunately, in our case, this breaks, because we use a forced-command, so we don't ever send a command, which causes SSH to always allocate TTY, which causes a variety of problems, not least of which is that stdout and stderr end up merged.

Now, it's pretty common for Aptible users to run commands in their container with the intention of using a TTY (by e.g. running aptible ssh bash), so we use a slightly different heuristic from SSH: we allocate TTY iif there's no input or output redirection going on.

End users can always override this behavior with the --force-tty option.

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cc @fancyremarker

[codecov-io]

Codecov Report

Merging #217 into master will increase coverage by 0.59%.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   90.55%   91.15%   +0.59%     
==========================================
  Files          43       44       +1     
  Lines        1440     1481      +41     
==========================================
+ Hits         1304     1350      +46     
+ Misses        136      131       -5

Impacted Files	Coverage Δ
spec/aptible/cli/subcommands/ssh_spec.rb	100% <100%> (ø)
lib/aptible/cli/subcommands/ssh.rb	100% <100%> (+30.43%)	:white_check_mark:
lib/aptible/cli/helpers/tunnel.rb	90.19% <ø> (-3.93%)	:x:

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