Correctly handle edge case were first template text is conditional.

[amcgregor]

The following example produces strangeness:

: def template
    : if False
        Hi.

This generates the following code (pre-1.0):

def template(*, _escape=_escape, _bless=_bless, _args=_args):
    if False:
        _buffer = []
        __w, __ws = _buffer.extend, _buffer.append

        __ws('            Hi.\n')

    yield "".join(_buffer)

The _buffer allocation being itself conditional results in problems. Lame, naive solution: have all template functions call ensure_buffer immediately.

[amcgregor]

Resolved by ensuring the buffer is immediately recreated upon flush, rather than relying on deferred reconstruction.