Detect buffering failures while writing rebase todo file

Detect buffering failures while writing rebase todo file

By routing writes through SafeBufferedOutputStream the caller can be
alerted to any flush at close failures while writing or appending to
the rebase todo script.

Switch the character encoding to be done at the line granularity, as
this is sufficiently long enough that encoding overheads will not be a
bottleneck, but short enough that the amount of temporary data will
not cause memory problems for the JVM.

Change-Id: Ice5ec10a7cbadc58486d481b92940056f9ffc43a