core: Add API (and standard concept for) content checksum

There are a few cases for knowing whether a commit has identical
content to another commit.  Some people want to do a "promotion workflow",
where the content of a commit on a tesitng branch is then "promoted"
to a production branch with `ostree commit --tree=ref`.

Another use case I just hit in rpm-ostree deals with
[jigdo](https://github.com/projectatomic/rpm-ostree/issues/1081) where we're
importing RPMs on both the client and server, and will be using the
content checksum, since the client/server cases inject different metadata
into the commit object.

Closes: https://github.com/ostreedev/ostree/issues/1315

Closes: #1449
Approved by: jlebon

tests/basic-test.sh

@@ -21,7 +21,7 @@
 
 set -euo pipefail
 
-echo "1..$((81 + ${extra_basic_tests:-0}))"
+echo "1..$((82 + ${extra_basic_tests:-0}))"
 
 CHECKOUT_U_ARG=""
 CHECKOUT_H_ARGS="-H"
@@ -755,6 +755,16 @@ assert_file_has_content show-output "Third commit"
 assert_file_has_content show-output "commit $checksum"
 echo "ok show full output"
 
+grep -E -e '^ContentChecksum' show-output > previous-content-checksum.txt
+cd $test_tmpdir/checkout-test2
+checksum=$($OSTREE commit ${COMMIT_ARGS} -b test4 -s "Another commit with different subject")
+cd ${test_tmpdir}
+$OSTREE show test4 | grep -E -e '^ContentChecksum' > new-content-checksum.txt
+if ! diff -u previous-content-checksum.txt new-content-checksum.txt; then
+    fatal "content checksum differs"
+fi
+echo "ok content checksum"
+
 cd $test_tmpdir/checkout-test2
 checksum1=$($OSTREE commit ${COMMIT_ARGS} -b test5 -s "First commit")
 checksum2=$($OSTREE commit ${COMMIT_ARGS} -b test5 -s "Second commit")