t1050-large: generate large files without dd

For some unknown reason, the dd on my Windows box segfaults randomly, but since recently, it does so much more often than it used to, which makes running the test suite burdensome. Use printf to write large files instead of dd. To emphasize that three of the large blobs are exact copies, use cp to allocate them. The new code makes the files a bit smaller, and they are not sparse anymore, but the tests do not depend on these properties. We do not want to use test-genrandom here (which is used to generate large files elsewhere in t1050), so that the files can be compressed well (which keeps the run-time short). The files are now large text files, not binary files. But since they are larger than core.bigfilethreshold they are diagnosed as binary by Git. For this reason, the 'git diff' tests that check the output for "Binary files differ" still pass. Signed-off-by: Johannes Sixt <j6t@kdbg.org> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>