libostree: Reformat bootloader titles

Use the pattern:

  $PRETTY_NAME [$COMMIT_VERSION] (ostree[:$OSNAME][:$DEPLOYMENT_INDEX])

$OSNAME is only shown if there are multiple values.

$COMMIT_VERSION refers to the version tag in the commit's metadata.

$DEPLOYMENT_INDEX is only shown if no $COMMIT_VERSION is available.

https://bugzilla.gnome.org/show_bug.cgi?id=739416