pull: Support specifying exact commit to pull via branch@commit

I don't know why we didn't do this a long time ago.  This extends the
pull API to allow grabbing a specific commit, and will set the branch
to it.  There's some support for this in the deploy engine, but there
are a lot of reasons to support it for raw pulls (such as subset
mirroring cases).

In fact I'm thinking we should also have the override-version logic
here too.

NOTE: One thing I debated here is inventing a new syntax on the
command line.  Git doesn't seem to have this functionality (probably
because it'd be rarely used). The '@' character at least doesn't
conflict with anything.

Anyways, I wanted this for some other test cases.  Without this,
writing tests that go between different commits is more awkward as one
must generate the content in one repo, then pull downstream, then
generate more content, then pull again.  But now I can just keep track
of commit IDs and do exactly what I want without synchronizing the
tests.

tests/pull-test.sh

@@ -72,6 +72,21 @@ $OSTREE show --print-detached-metadata-key=SIGNATURE main > main-meta
 assert_file_has_content main-meta "HANCOCK"
 echo "ok pull detached metadata"
 
+cd ${test_tmpdir}
+mkdir parentpullrepo
+${CMD_PREFIX} ostree --repo=parentpullrepo init --mode=archive-z2
+${CMD_PREFIX} ostree --repo=parentpullrepo remote add --set=gpg-verify=false origin file://$(pwd)/ostree-srv/gnomerepo
+parent_rev=$(ostree --repo=ostree-srv/gnomerepo rev-parse main^)
+rev=$(ostree --repo=ostree-srv/gnomerepo rev-parse main)
+${CMD_PREFIX} ostree --repo=parentpullrepo pull origin main@${parent_rev}
+${CMD_PREFIX} ostree --repo=parentpullrepo rev-parse origin:main > main.txt
+assert_file_has_content main.txt ${parent_rev}
+${CMD_PREFIX} ostree --repo=parentpullrepo fsck
+${CMD_PREFIX} ostree --repo=parentpullrepo pull origin main
+${CMD_PREFIX} ostree --repo=parentpullrepo rev-parse origin:main > main.txt
+assert_file_has_content main.txt ${rev}
+echo "ok pull specific commit"
+
 cd ${test_tmpdir}
 repo_init
 ${CMD_PREFIX} ostree --repo=repo pull origin main