mail: use procutil.shellsplit instead of bytes.split to parse command

mail: use procutil.shellsplit instead of bytes.split to parse command

Differential Revision: https://phab.mercurial-scm.org/D7541

--- a/mercurial/mail.py	Thu Dec 05 16:18:44 2019 -0500
+++ b/mercurial/mail.py	Mon Dec 02 14:44:26 2019 +0100
@@ -197,7 +197,7 @@
         raise error.Abort(
             b'%s %s'
             % (
-                os.path.basename(program.split(None, 1)[0]),
+                os.path.basename(procutil.shellsplit(program)[0]),
                 procutil.explainexit(ret),
             )
         )