PowerShell Core on Linux support

[theaquamarine]

Issues I've found running posh-git on OS X:

• Path separators are a known issue in PowerShell on Linux: only forward slashes are supported (https://github.com/PowerShell/PowerShell/blob/master/docs/KNOWNISSUES.md#directly). profile.example.ps1 uses .\posh-git rather than Join-Path . posh-git. Failing that, swapping to forward slash would work as they are supported everywhere.
• Administrator check in GitPrompt.ps1:98 doesn't work as [Security.Principal.WindowsIdentity]::GetCurrent() is unsupported.
• GitUtils.ps1:226 doesn't work for setting env variables, as [EnvironmentVariableTarget] is unavailable. We use Process, which is the default, so could skip this.
• The prompt shows up twice. This isn't actually our fault, there's an issue with using Write-Host in the prompt on OS X/Linux. Tracked upstream at Powershell Prompt Double Execution PowerShell/PowerShell#1897
• Actual ssh-agent support could probably be better. ssh-agent always runs OS X 10.5+ and uses Keychain as storage so I'm not sure how much posh-git actually needs to manage agent/keys on OS X.

[jonathankarsh]

Can confirm similar issues exist on Ubuntu 14.04.

[andyleejordan]

Path separators actually shouldn't be an issue. PowerShell on Linux translates backslashes to forward slashes to be "slash agnostic"; what this prevents is a backslash as a literal character in a path name due to the inability to escape it. But your use of \ as a / should be just fine.

Anything using Security.Principal.WindowsIdentity is just (expectedly) not going to work on Linux.

You honestly may just want to wait for EnvironmentVariableTarget. It's been added back to CoreFX. However, if you note the date, that was July 5th, and the stable release that PowerShell is using was published June 27th. So when .NET Core drops their next big release and PowerShell picks it up, this problem just goes away. (And then GetCurrent can turn into [Environment]::UserName probably.)

Double prompting is a strange bug; I believe @ealexjordan is working on it.

[dahlbyk]

Thanks, @andschwa!

Double prompting is a strange bug; I believe @ealexjordan is working on it.

Don't spend too much time there, at least for our benefit: as of #304 we won't need to Write-Host in prompt.

[rkeithhill]

Issues 2 and 3 have been addressed. Issue 1 "shouldn't" be an issue for PowerShell commands but anytime a path is passed to an external application, the path sep char probably needs to be correct for the platform. Regarding issue 4, that is a known PSReadLine issue. Issue 5 needs further investigation.

And as @dahlbyk mentions, there is an issue with the use of Write-Host in Write-Prompt. On Window 10 AU and Linux/macOS, folks are going to want to start using ANSI esc sequences to colorize their prompt text. There should be a way to allow that while still maintaining the old way for back compat (or for hosts that don't support ANSI escape seqs).

[dahlbyk]

Issues 2 and 3 have been addressed.

By #312 and #318, respectively.

Are there any specific spots that Issue 1 (path separators) were an issue? #325 did make a minor change along these lines.

Issue 4 should be resolved by #304.

Issue 5 is related to #295. Are there any environment variables set by ssh-agent on macOS (e.g. SSH_AGENT_PID)?