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

ci/run-build-and-tests.sh

@@ -17,6 +17,7 @@ linux-gcc)
 	export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 	;;
 linux-TEST-vars)
+	export OPENSSL_SHA1_UNSAFE=YesPlease
 	export GIT_TEST_SPLIT_INDEX=yes
 	export GIT_TEST_MERGE_ALGORITHM=recursive
 	export GIT_TEST_FULL_IN_PACK_ARRAY=true