sshpeer: allow for additional environment passing to ssh exe We already have the ability to customize the ssh command line arguments, let's add the ability to customize its environment as well. Example use-case is ssh.exe from Git on Windows. If `HOME` enviroment variable is present and has some non-empty value, ssh.exe will try to access that location for some stuff (for example, it seems for resolving `~` in `.ssh/config`). Git for Windows seems to sometimess set this variable to the value of `/home/username` which probably works under Git Bash, but does not work in a native `cmd.exe` or `powershell`. Whatever the root cause, setting `HOME` to be an empty string heals things. Therefore, some distributors might want to set `sshenv.HOME=` in the configuration (seems less intrusive that forcing everyone to tweak their env). Test Plan: - rt Differential Revision: https://phab.mercurial-scm.org/D1683