t5000 on Windows: do not mistake "sh.exe" as "sh"

t5000 on Windows: do not mistake "sh.exe" as "sh"

In their effort to emulate POSIX as close as possible, the MSYS tools
and Cygwin treat the file name "foo.exe" as "foo" when the latter is
asked for, but not present, but the former is present.

Following this rule, 'cp /bin/sh a/bin' actually copies the file
/bin/sh.exe, so that we now have a/bin/sh.exe in the repository. This
difference did not matter in the tests in the past because we were only
interested in the equality of contents generated in various ways. But
recently added tests check file names, in particular, the presence of
"a/bin/sh". This test fails on Windows, as we do not have a file by this
name, but "a/bin/sh.exe".

Use test-genrandom to generate the large binary file in the repository
under the expected name.

We could change the guilty line to 'cat /bin/sh >a/bin/sh', but it is
better for test reproducibility to ensure that the test data is the same
across platforms, which test-genrandom can guarantee.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>