core: Add standard SOURCE_TITLE metadata key

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