Improve pageant process handling

[GrahamTheCoder]

Don't wait indefinitely for pageant (given that it's a service-like process that adds a tray icon and waits for input), and don't error when there are already 2 pageant processes.

Hanging bug
This is the old behaviour I observed (that I believe f28bb54 removes):
Start state:

• I am on Windows 10 with pageant 0.63 running. GitUtils runs as part of my powershell profile.
• I start powershell.exe and run $env:GIT_SSH = "plink"
• I kill any pageant.exe that is running

Repro:

• I run Start-Process powershell.exe which creates a new powershell window, starts pageant, then hangs forever (waiting for it to return).
• I close the hanging powershell window, pageant stays running
• I run Start-Process powershell.exe in my original powershell window which starts fine since pageant is already running

Multiple-pageant bug
When using GitExtensions on my machine I end up with two pageant processes (one is created by ShimGen), 99021d4 fixes the powershell error thrown in Start-SshAgent due to having an array returned. Stop-SshAgent also uses this, so should work with multiple pageant processes, but you need to call Stop-SshAgent once for each running SshAgent process which could be better, but is an improvement over doing nothing and erroring.

[theaquamarine]

Do we actually end up waiting for pageant? It always returns immediately for me, even if it's asking for a key to be unlocked. Some quick testing timing the options for launching pageant looks like there's not much difference.

[dahlbyk]

Whitespace czar has come to visit - extra space here.

[dahlbyk]

Is the waiting indefinitely an artifact of the instance already running?

[theaquamarine]

I don't think makes a difference, since an existing process would have caused Start-SshAgent to exit by then. Attempting to launch pageant while it's already running manually (without specifying a key to add) causes it to immediately return as normal, but with a "Pageant is already running" alert.

[GrahamTheCoder]

I've updated the description with an explanation of what I'm seeing.

[dahlbyk]

I'm not a Pageant user, so I'll defer to whatever you come up with. I don't see a down-side to switching from & to Start-Process.

[GrahamTheCoder]

I briefly thought it might not work in Powershell 2.0 and below due to not being in these docs. Turns out the docs are just somewhere else.

[theaquamarine]

No downsides and fixes issues, let's get this merged.

[dahlbyk]

💯