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
Fix two nits about DirCacheEntry constructors

Explicitly pass STAGE_0 when creating a DirCacheEntry from String.
This matches the immediate next constructor that accepts the int
stage argument better, making the code easier to read.

Fix a weird line break where the comma was orphaned by itself.

Change-Id: Icf0970dd02a63877f9e41b51b982b0265e8b8887
Merge changes Icf0970dd,Ice5ec10a into stable-3.4

* changes:
  Fix two nits about DirCacheEntry constructors
  Detect buffering failures while writing rebase todo file