posix: tiptoe around tempfile module more delicately

Some of the values inside the tempfile calls here are str on Python 3,
so we've got to pass str in. Use fsdecode to work around the issue.

mercurial/posix.py

@@ -230,13 +230,16 b' def checklink(path):'
         else:
             checkdir = path
             cachedir = None
-        name = tempfile.mktemp(dir=checkdir, prefix='checklink-')
+        fscheckdir = pycompat.fsdecode(checkdir)
+        name = tempfile.mktemp(dir=fscheckdir,
+                               prefix=r'checklink-')
+        name = pycompat.fsencode(name)
         try:
             fd = None
             if cachedir is None:
-                fd = tempfile.NamedTemporaryFile(dir=checkdir,
-                                                 prefix='hg-checklink-')
-                target = os.path.basename(fd.name)
+                fd = tempfile.NamedTemporaryFile(dir=fscheckdir,
+                                                 prefix=r'hg-checklink-')
+                target = pycompat.fsencode(os.path.basename(fd.name))
             else:
                 # create a fixed file to link to; doesn't matter if it
                 # already exists.