uescape: also encode non-printable char under 128

We were assuming everything under 128 was printable ascii, but there are a lot
of control characters in that range that can't simply be included in json and
other targets. We forcibly encode everything under 32, because they are either
control char or oddly printable (like tab or line ending).

We also add the hypothesis-powered test that caught this.

mercurial/templatefilters.py

@@ -219,7 +219,7 b' def json(obj):'
         raise TypeError('cannot encode type %s' % obj.__class__.__name__)
 
 def _uescape(c):
-    if ord(c) < 0x80:
+    if 0x20 <= ord(c) < 0x80:
         return c
     else:
         return '\\u%04x' % ord(c)

tests/test-template-engine.t

@@ -44,4 +44,17 b''
   0 97e5f848f0936960273bbf75be6388cd0350a32b -1 0000000000000000000000000000000000000000
   -1 0000000000000000000000000000000000000000 -1 0000000000000000000000000000000000000000
 
+Fuzzing the unicode escaper to ensure it produces valid data
+
+#if hypothesis
+
+  >>> from hypothesishelpers import *
+  >>> import mercurial.templatefilters as tf
+  >>> import json
+  >>> @check(st.text().map(lambda s: s.encode('utf-8')))
+  ... def testtfescapeproducesvalidjson(text):
+  ...     json.loads('"' + tf.jsonescape(text) + '"')
+
+#endif
+
   $ cd ..