core: Add standard SOURCE_TITLE metadata key

This is a freeform string useful to track/display when a commit is "derived"
from some other format.  For example, in the rpm-ostree test we make a
`vmcheck` ref that conceptually overlays the default ref like
`fedora-atomic:fedora/26/x86_64/atomic-host`.

My current patch sets the source title to e.g.
"Dev overlay on fedora-atomic:fedora/26/x86_64/atomic-host".

Another case I'm working on now is importing OCI images to use
as host images.  For that case, the source title is
With this patch we could then set the original OCI image name + tag
as the source name, like:
"oci:cgwalters/demo-custom-fedora-atomic-host:26".

Closes: #1296
Approved by: jlebon

src/libostree/ostree-core.h

@@ -228,6 +228,23 @@ typedef enum {
  * Since: 2017.7
  */
 #define OSTREE_COMMIT_META_KEY_ENDOFLIFE "ostree.endoflife"
+/**
+ * OSTREE_COMMIT_META_KEY_SOURCE_TITLE:
+ *
+ * GVariant type `s`. This should hold a relatively short single line value
+ * containing a human-readable "source" for a commit, intended to be displayed
+ * near the origin ref.  This is particularly useful for systems that inject
+ * content into an OSTree commit from elsewhere - for example, generating from
+ * an OCI or qcow2 image. Or if generating from packages, the enabled repository
+ * names and their versions.
+ *
+ * Try to keep this key short (e.g. < 80 characters) and human-readable; if you
+ * desire machine readable data, consider injecting separate metadata keys.
+ *
+ * Since: 2017.13
+ */
+#define OSTREE_COMMIT_META_KEY_SOURCE_TITLE "ostree.source-title"
+
 /**
  * OSTREE_COMMIT_META_KEY_REF_BINDING:
  *