xchangelog: try repo db if template not available

[tornaria]

When either

1. the template is not available (maybe repo is not checked out)
2. the template does not define changelog=

This will try (in order) to get the changelog property from the local db / remote repo.

Usecases:

• user doesn't have the repo checked out, get the changelog from the package (first from local, saves one network roundtrip for installed packages, then from remote so it works for uninstalled packages)
• maintainer has an updated version of package locally built and installed in system; this updated package added a changelog property but the template in the checked out repo doesn't have the changelog property yet.

Arguably, the property from locally installed package should take precedence (this is important if the changelog was changed) but I didn't want to alter the behaviour for anyone already using this tool.

cc: @mhmdanas @classabbyamp

[triallax]

Suggested change

	echo "changelog not available" >&2
	echo "No changelog found" >&2