test-lib: teach test_seq the -f option

The "seq" tool has a "-f" option to produce printf-style formatted
lines. Let's teach our test_seq helper the same trick. This lets us get
rid of some shell loops in test snippets (which are particularly verbose
in our test suite because we have to "|| return 1" to keep the &&-chain
going).

This converts a few call-sites I found by grepping around the test
suite. A few notes on these:

- In "seq", the format specifier is a "%g" float. Since test_seq only
supports integers, I've kept the more natural "%d" (which is what
these call sites were using already).

- Like "seq", test_seq automatically adds a newline to the specified
format. This is what all callers are doing already except for t0021,
but there we do not care about the exact format. We are just trying
to printf a large number of bytes to a file. It's not worth
complicating other callers or adding an option to avoid the newline
in that caller.

- Most conversions are just replacing a shell loop (which does get rid
of an extra fork, since $() requires a subshell). In t0612 we can
replace an awk invocation, which I think makes the end result more
readable, as there's less quoting.

- In t7422 we can replace one loop, but sadly we have to leave the
loop directly above it. This is because that earlier loop wants to
include the seq value twice in the output, which test_seq does not
support (nor does regular seq). If you run:

test_seq -f "foo-%d %d" 10

the second "%d" will always be the empty string. You might naively
think that test_seq could add some extra arguments, like:

# 3 ought to be enough for anyone...
printf "$fmt\n" "$i "$i" $i"

but that just triggers printf to format multiple lines, one per
extra set of arguments.

So we'd have to actually parse the format string, figure out how
many "%" placeholders are there, and then feed it that many
instances of the sequence number. The complexity isn't worth it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>