bin/prune: Add --only-branch

In 5c940987e7 / #646 we
added `--retain-branch-depth`; this adds a symmetric
`--only-branch` for the case where a repo owner just
wants to prune a specific branch.

The implementation here is pretty straightforward; we
just walk all refs and inject the equivalent of
`--retain-branch-depth=$ref=-1` if they're *not* in
`--only-branch`.

Closes: https://github.com/ostreedev/ostree/issues/1115

Closes: #1127
Approved by: jlebon

tests/test-prune.sh

@@ -25,7 +25,7 @@ skip_without_user_xattrs
 
 setup_fake_remote_repo1 "archive"
 
-echo '1..5'
+echo '1..12'
 
 cd ${test_tmpdir}
 mkdir repo
@@ -235,3 +235,54 @@ assert_repo_has_n_commits repo 9
 $OSTREE fsck
 
 echo "ok retain branch depth (alone)"
+
+# Test --only-branch with --depth=0; this should be exactly identical to the
+# above with a result of 9.
+reinitialize_datesnap_repo
+${CMD_PREFIX} ostree --repo=repo prune --only-branch=dev --depth=0
+assert_repo_has_n_commits repo 9
+$OSTREE fsck
+echo "ok --only-branch --depth=0"
+
+# Test --only-branch with --depth=1; should just add 1 to the above, for 10.
+reinitialize_datesnap_repo
+${CMD_PREFIX} ostree --repo=repo prune --only-branch=dev --depth=1
+assert_repo_has_n_commits repo 10
+echo "ok --only-branch --depth=1"
+
+# Test --only-branch with all branches
+reinitialize_datesnap_repo
+${CMD_PREFIX} ostree --repo=repo prune --only-branch=dev --only-branch=stable --depth=0
+assert_repo_has_n_commits repo 2
+reinitialize_datesnap_repo
+${CMD_PREFIX} ostree --repo=repo prune --only-branch=dev --only-branch=stable --depth=1
+assert_repo_has_n_commits repo 4
+echo "ok --only-branch (all) --depth=1"
+
+# Test --only-branch and --retain-branch-depth overlap
+reinitialize_datesnap_repo
+${CMD_PREFIX} ostree --repo=repo prune --only-branch=dev --only-branch=stable --depth=0 \
+                     --retain-branch-depth=stable=2
+assert_repo_has_n_commits repo 4
+echo "ok --only-branch and --retain-branch-depth overlap"
+
+# Test --only-branch and --retain-branch-depth together
+reinitialize_datesnap_repo
+${CMD_PREFIX} ostree --repo=repo prune --only-branch=dev --depth=0 --retain-branch-depth=stable=2
+assert_repo_has_n_commits repo 4
+echo "ok --only-branch and --retain-branch-depth together"
+
+# Test --only-branch with --keep-younger-than; this should be identical to the test
+# above for --retain-branch-depth=stable=-1
+reinitialize_datesnap_repo
+${CMD_PREFIX} ostree --repo=repo prune --only-branch=stable --keep-younger-than="1 week ago"
+assert_repo_has_n_commits repo 11
+echo "ok --only-branch --keep-younger-than"
+
+# Test --only-branch with a nonexistent ref
+reinitialize_datesnap_repo
+if ${CMD_PREFIX} ostree --repo=repo prune --only-branch=BACON 2>err.txt; then
+    fatal "we pruned BACON?"
+fi
+assert_file_has_content err.txt "Refspec.*BACON.*not found"
+echo "ok --only-branch=BACON"