vcs: simplify nodes kind handling

Avoid pytype's very reasonable confusion over the _kind handling.

Node.kind is actually only ever set in __init__, so there is not much need for
a paranoid setter.

diff: fix two-way diff for added or removed files

Viewing the two-way diff of an added file gives following exception:

File "_base_root_html", line 211, in render_body

File "_base_base_html", line 42, in render_body

File "files_diff_2way_html", line 197, in render_main

File ".../kallithea/lib/vcs/nodes.py", line 411, in is_binary
return b'\0' in self.content
TypeError: 'in <string>' requires string as left operand, not bytes

At this point, self.content was '' (empty string).

Commit 9203621cae03a03bbf8cfb4b667daa656780a93e made that node content is
always bytes, but this also had to be reflected in EmptyChangeset.

Add a basic test that catches this behavior.

py3: add b'' annotations in some places where they will be needed later

Mostly entirely trivial adding of b prefix that is a ignored for py2 ... and
also a bit of related trivial reformatting/refactorings.

py3: use explicit octal literals

From 2to3 numliterals.

codingstyle: trivial whitespace fixes

Reported by flake8.

vcs: remove unused code and test of it

Neither git nor mercurial nodes have a _mimetype attribute. Only
test_nodes.test_mimetype monkey patches it and that to me seems just wrong.
Thus, remove that part and fix the test with some usefull assertions.

Found by pylint.

tests: fix test_nodes.py and test_files.py which fail on windows due to mimetype differences

The mimetype of files may differ on different platforms (e.g. the mimetype of
.py files on windows is not text/x-python but text/plain). Therefore, compare
the mimetype to the value the platform uses.

remove vestiges of Python 2.5 support

We only support Python 2.6 and 2.7; hence we do not need to import
with-statement support from __future__.

Second step in two-part process to rename directories.
This is the actual directory rename.