[PATCH] t1050-large: replace dd by test-genrandom

For some unknown reason, the dd on my Windows box segfaults every now and than, but since recently, it does so much more often than it used to, which makes running the test suite burdensome.

Get rid of four invocations of dd and use test-genrandom instead.

The new code does change some properties of the generated files:

 - They are a bit smaller.
 - They are not sparse anymore.
 - They do not compress well anymore.
 - The smaller of the four files is now a prefix of the larger.

Fortunately, the tests do not depend on these properties, which would have a big influence on the size of the generated pack files. There *is* a test in t1050 that checks the size of pack files generated from large blobs, but it runs in its own repository with its own set of files (that are already generated with test-genrandom!).

To emphasize that three of the large blobs are exact copies, use cp to allocate them.

While we are here, replace cmp with test_cmp_bin to document the binary-ness of the comparison, which was hinted at by a comment, but not stated explicitly.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
---
 I won't mind if the first paragraph of the proposed commit message is
 removed, but without the motivation, this commit reads as if it were
 merely code churn.

 The existing test-genrandom invocations look like this:

   test-genrandom "a" $(( 66 * 1024 )) >mid1

 I chose not to mimick this style because being precise with the file
 size is not important for the files generated here.

 t/t1050-large.sh | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)