Merge branch 'ps/weak-sha1-for-tail-sum-fix'

An earlier "csum-file checksum does not have to be computed with
sha1dc" topic had a few code paths that had initialized an
implementation of a hash function to be used by an unmatching hash
by mistake, which have been corrected.

* ps/weak-sha1-for-tail-sum-fix:
  ci: exercise unsafe OpenSSL backend
  builtin/fast-import: fix segfault with unsafe SHA1 backend
  bulk-checkin: fix segfault with unsafe SHA1 backend

diff --cc builtin/fast-import.c
Simple merge

diff --cc bulk-checkin.c
Simple merge

diff --cc ci/run-build-and-tests.sh
Simple merge