phabricator: correct the default value of `phabhunk.corpus`

There's only one caller to this constructor (which does provide this argument),
and no direct assignments, so there's no runtime bug here.  However, when pytype
is tricked into using the standard `attr` package instead of the vendored
version, it flags this because bytes is passed to the one constructor
invocation.

Tricking pytype into using the standard package will generate many more type
hints, noteably around `@attr.s` decorated things.

hgext/phabricator.py

@@ -698,7 +698,7 b' class phabhunk(dict):'
     oldLength = attr.ib(default=0)  # camelcase-required
     newOffset = attr.ib(default=0)  # camelcase-required
     newLength = attr.ib(default=0)  # camelcase-required
-    corpus = attr.ib(default='')
+    corpus = attr.ib(default=b'')
     # These get added to the phabchange's equivalents
     addLines = attr.ib(default=0)  # camelcase-required
     delLines = attr.ib(default=0)  # camelcase-required