git-svnimport: Don't assume that copied files haven't changed

Don't assume that a file that SVN claims was copied from somewhere
else is bit-for-bit identical with its parent, since SVN allows
changes to copied files before they are committed.

Without this fix, such copy-modify-commit operations causes the
imported file to lack the "modify" part -- that is, we get subtle data
corruption.

Signed-off-by: Karl Hasselström <kha@treskal.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>