add "ok=sigpipe" to test_must_fail and use it to fix flaky tests

add "ok=sigpipe" to test_must_fail and use it to fix flaky tests

t5516 "75 - deny fetch unreachable SHA1, allowtipsha1inwant=true" is
flaky in the following case:
1. remote upload-pack finds out "not our ref"
2. remote sends a response and closes the pipe
3. fetch-pack still tries to write commands to the remote upload-pack
4. write call in wrapper.c dies with SIGPIPE

The test is flaky because the sending fetch-pack may or may
not have finished writing its output by step (3). If it did,
then we see a closed pipe on the next read() call. If it
didn't, then we get the SIGPIPE from step (4) above. Both
are fine, but the latter fools test_must_fail.

t5504 "9 - push with transfer.fsckobjects" is flaky, too, and returns
SIGPIPE once in a while. I had to remove the final "To dst..." output
check because there is no output if the process dies with SIGPIPE.

Accept such a death-with-sigpipe also as OK when we are expecting a
failure.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>