[henriquejsza]

Fix #4736

Motivation

FileInfo.moveToNewLocation copies the ledger index file to a new location with
FileChannel.transferTo:

long written = 0;
while (written < size) {
    long count = fc.transferTo(written, size, newFc);
    ...
    written += count;
}
if (written <= 0 && size > 0) {
    throw new IOException("Copying to new location " + rlocFile + " failed");
}

transferTo may transfer fewer bytes than requested. Because the count passed to
every call is size (not the number of bytes still to copy), a call after a
partial transfer reads from position written for up to size more bytes. When
the source file holds more than size bytes, that copies past the requested
range, so the new location can end up with more than size bytes. This matters
in practice: moveToNewLocation is called with getSizeSinceLastWrite(), which
can be smaller than the actual file size.

The post-loop written <= 0 && size > 0 check reported in the issue is
unreachable: when size > 0 the loop runs at least once and either throws or
increments written to a positive value.

Changes

• Pass the remaining byte count (size - written) to transferTo on each
  iteration so exactly size bytes are copied.
• Extract the copy loop into a small package-private transferFully helper and
  remove the unreachable post-loop check.
• Add FileInfoTest covering a partial transfer, asserting that exactly size
  bytes are copied.