Escape with double quotes instead of single quotes

This takes us away from the standard shlex implementation. It is still
not what you want on Windows (backslashes will not be understood),
however it should do what you want in more situations (for example it
supports spaces).

scp.py

@@ -14,21 +14,25 @@
 import types
 
 
-# this is quote from the shlex module, added in py3.3
-_find_unsafe = re.compile(br'[^\w@%+=:,./~-]').search
+safe_shell_chars = set("ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+                       "abcdefghijklmnopqrstuvwxyz"
+                       "0123456789"
+                       "-+=/:.,%_")
 
 
 def _sh_quote(s):
-    """Return a shell-escaped version of the string `s`."""
-    if not s:
-        return b""
-    if _find_unsafe(s) is None:
+    r"""Given bl"a, returns "bl\\"a".
+    """
+    if isinstance(s, bytes):
+        s = s.decode('utf-8')
+    if not s or any(c not in safe_shell_chars for c in s):
+        return '"%s"' % (s.replace('\\', '\\\\')
+                          .replace('"', '\\"')
+                          .replace('`', '\\`')
+                          .replace('$', '\\$'))
+    else:
         return s
 
-    # use single quotes, and put single quotes into double quotes
-    # the string $'b is then quoted as '$'"'"'b'
-    return b"'" + s.replace(b"'", b"'\"'\"'") + b"'"
-
 
 # Unicode conversion functions; assume UTF-8