t1050-large: generate large files without dd

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>