extdiff: avoid double backslashes in the displayed tool path on Windows

This shows the tool path in the help, and changed in 67b180c0e263.  uirepr()
already does the same thing, but that undoes the mangling in its call to repr().

hgext/extdiff.py

@@ -383,8 +383,9 b' class savedcmd(object):'
 
     def __init__(self, path, cmdline):
         # We can't pass non-ASCII through docstrings (and path is
-        # in an unknown encoding anyway)
-        docpath = stringutil.escapestr(path)
+        # in an unknown encoding anyway), but avoid double separators on
+        # Windows
+        docpath = stringutil.escapestr(path).replace(b'\\\\', b'\\')
         self.__doc__ %= {r'path': pycompat.sysstr(stringutil.uirepr(docpath))}
         self._cmdline = cmdline