Treat RawText of binary data as file with one single line.

Treat RawText of binary data as file with one single line.

This avoids executing mergeAlgorithm.merge on binary data, which is
unlikely to be useful.

Arguably, binary data should not make it to
ResolveMerger#contentMerge, but this approach has the following
advantages:

* binary detection is exact, since it doesn't only look at the start
  of the blob.

* it is cheap, as we have to iterate over the bytes anyway to find
  '\n'.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Change-Id: I424295df1dc60a719859d9d7c599067891b15792