Use plenary.job for call_sync

[eternity74]

This change helps to capture output of the command correctly. Current neogit.process implementation doesn't capture stdout correctly regarding empty lines and EOF.

This would help to remove the last empty line issue on #1108

[CKolkey]

seems reasonable, but I'll revisit the job handling when 0.10 drops and we get the vim.system api. I'd like to avoid wildly different implementations for sync and async calls, too

[CKolkey]

Going to shelve this as it's more likely we'll use vim.system instead