mail: don't complain about a multi-word email.method

I want to be able to set email.method to "ssh relay /usr/sbin/sendmail"
without needing an extra trivial shell script.
This works fine since we pass the full command to a shell, except for
validateconfig trying to find it in $PATH.

Differential Revision: https://phab.mercurial-scm.org/D7542

mercurial/mail.py

@@ -260,9 +260,11 b' def validateconfig(ui):'
                 )
             )
     else:
-        if not procutil.findexe(method):
+        command = procutil.shellsplit(method)
+        command = command[0] if command else b''
+        if not (command and procutil.findexe(command)):
             raise error.Abort(
-                _(b'%r specified as email transport, but not in PATH') % method
+                _(b'%r specified as email transport, but not in PATH') % command
             )