git_remote_helpers: fix input when running under Python 3

Although 2to3 will fix most issues in Python 2 code to make it run under
Python 3, it does not handle the new strict separation between byte
strings and unicode strings.  There is one instance in
git_remote_helpers where we are caught by this, which is when reading
refs from "git for-each-ref".

Fix this by operating on the returned string as a byte string rather
than a unicode string.  As this method is currently only used internally
by the class this does not affect code anywhere else.

Note that we cannot use byte strings in the source as the 'b' prefix is
not supported before Python 2.7 so in order to maintain compatibility
with the maximum range of Python versions we use an explicit call to
encode().

Signed-off-by: John Keeping <john@keeping.me.uk>
git_remote_helpers: Force rebuild if python version changes

When different version of python are used to build via distutils, the
behaviour can change.  Detect changes in version and pass --force in
this case.

Signed-off-by: John Keeping <john@keeping.me.uk>
git_remote_helpers: Use 2to3 if building with Python 3

Using the approach detailed on the Python wiki[1], run 2to3 on the code
as part of the build if building with Python 3.

The code itself requires no changes to convert cleanly.

[1] http://wiki.python.org/moin/PortingPythonToPy3k

Signed-off-by: John Keeping <john@keeping.me.uk>
svn-fe: allow svnrdump_sim.py to run with Python 3

The changes to allow this script to run with Python 3 are minimal and do
not affect its functionality on the versions of Python 2 that are
already supported (2.4 onwards).

Signed-off-by: John Keeping <john@keeping.me.uk>
git-remote-testpy: hash bytes explicitly

Under Python 3 'hasher.update(...)' must take a byte string and not a
unicode string.  Explicitly encode the argument to this method as UTF-8
so that this code works under Python 3.

This moves the required Python version forward to 2.0.

Signed-off-by: John Keeping <john@keeping.me.uk>
git-remote-testpy: don't do unbuffered text I/O

Python 3 forbids unbuffered I/O in text mode.  Change the reading of
stdin in git-remote-testpy so that we read the lines as bytes and then
decode them a line at a time.

This allows us to keep the I/O unbuffered in order to avoid
reintroducing the bug fixed by commit 7fb8e16 (git-remote-testgit: fix
race when spawning fast-import).

Signed-off-by: John Keeping <john@keeping.me.uk>
git-remote-testpy: call print as a function

This is harmless in Python 2, which sees the parentheses as redundant
grouping, but is required for Python 3.  Since this is the only change
required to make this script just run under Python 3 without needing
2to3 it seems worthwhile.

The case of an empty print must be handled specially because in that
case Python 2 will interpret '()' as an empty tuple and print it as
'()'; inserting an empty string fixes this.

Signed-off-by: John Keeping <john@keeping.me.uk>
Makefile: Run 2to3 on Python scripts

If PYTHON_PATH points at a Python 3 interpreter, run 2to3 on Python
scripts as they are being built.

The user is free to specify a different value for PYTHON_2TO3 if the
default is incorrect, but we always overwrite this when building under
Python 2.

Signed-off-by: John Keeping <john@keeping.me.uk>