ui: use "procutil.shellsplit" to parse command

A commandline containing a space ('"C:\\Program Files\\bar.exe" "..."')
must not simply split at whitespace, instead quoting has to be taken into
account. Use "shlex.split()" to parse it instead.

This can improve the error message if we fail to launch a user
configured
editor which does not exist. Consider

     [ui]
     editor = "C:\Program Files\editor\editor.exe"

where the path does not exist. "hg histedit" currently aborts with

> Abort: edit failed: Program exited with status 1

here "Program" is not part of the message but the name of the program
that failed (i.e. `basename("C:\\Program ")`). With this change the message
instead reads

> Abort: edit failed: C:\Program Files\editor\editor.exe exited with
> status 1

which is also not ideal since infact "cmd.exe" exited with code 1, not
the editor. But the real error message ("File not found") gets swallowed by
`procutil` and including the correct path improves the error message
nevertheless.

mercurial/ui.py

@@ -1868,7 +1868,7 b' class ui(object):'
             rc = self._runsystem(cmd, environ=environ, cwd=cwd, out=out)
         if rc and onerr:
             errmsg = b'%s %s' % (
-                os.path.basename(cmd.split(None, 1)[0]),
+                procutil.shellsplit(cmd)[0],
                 procutil.explainexit(rc),
             )
             if errprefix: