Ensure RawText closes the FileInputStream when read is complete

Rather than implementing the file reading logic ourselves, and
wind up leaking the FileInputStream's file descriptor until the
next GC, use IO.readFully(File) which wraps the read loop inside
of a try/finally to ensure the stream is closed before it exits.

Change-Id: I85a3fe87d5eff88fa788962004aebe19d2e91bb4
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Reviewed-by: Roland Grunberg <rgrunber@redhat.com>