sshpeer: allow for additional environment passing to ssh exe

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

--- a/mercurial/sshpeer.py	Wed Dec 13 17:03:39 2017 -0800
+++ b/mercurial/sshpeer.py	Thu Dec 14 14:31:57 2017 +0000
@@ -136,6 +136,8 @@
 
         sshcmd = self.ui.config("ui", "ssh")
         remotecmd = self.ui.config("ui", "remotecmd")
+        sshaddenv = dict(self.ui.configitems("sshenv"))
+        sshenv = util.shellenviron(sshaddenv)
 
         args = util.sshargs(sshcmd, self._host, self._user, self._port)
 
@@ -144,11 +146,11 @@
                 util.shellquote("%s init %s" %
                     (_serverquote(remotecmd), _serverquote(self._path))))
             ui.debug('running %s\n' % cmd)
-            res = ui.system(cmd, blockedtag='sshpeer')
+            res = ui.system(cmd, blockedtag='sshpeer', environ=sshenv)
             if res != 0:
                 self._abort(error.RepoError(_("could not create remote repo")))
 
-        self._validaterepo(sshcmd, args, remotecmd)
+        self._validaterepo(sshcmd, args, remotecmd, sshenv)
 
     # Begin of _basepeer interface.
 
@@ -180,7 +182,7 @@
 
     # End of _basewirecommands interface.
 
-    def _validaterepo(self, sshcmd, args, remotecmd):
+    def _validaterepo(self, sshcmd, args, remotecmd, sshenv=None):
         # cleanup up previous run
         self._cleanup()
 
@@ -196,7 +198,7 @@
         # no buffer allow the use of 'select'
         # feel free to remove buffering and select usage when we ultimately
         # move to threading.
-        sub = util.popen4(cmd, bufsize=0)
+        sub = util.popen4(cmd, bufsize=0, env=sshenv)
         self._pipeo, self._pipei, self._pipee, self._subprocess = sub
 
         self._pipei = util.bufferedinputpipe(self._pipei)

--- a/tests/test-ssh.t	Wed Dec 13 17:03:39 2017 -0800
+++ b/tests/test-ssh.t	Thu Dec 14 14:31:57 2017 +0000
@@ -596,3 +596,21 @@
   abort: no suitable response from remote hg!
   (Please see http://company/internalwiki/ssh.html)
   [255]
+
+test that custom environment is passed down to ssh executable
+  $ cat >>dumpenv <<EOF
+  > #! /bin/sh
+  > echo \$VAR >&2
+  > EOF
+  $ chmod +x dumpenv
+  $ hg pull ssh://something --config ui.ssh="./dumpenv"
+  pulling from ssh://something/
+  remote: 
+  abort: no suitable response from remote hg!
+  [255]
+  $ hg pull ssh://something --config ui.ssh="./dumpenv" --config sshenv.VAR=17
+  pulling from ssh://something/
+  remote: 17
+  abort: no suitable response from remote hg!
+  [255]
+