Expand manifest-provision to also supply changelog files

[mikepurvis]

I'm not sure what a reasonable CLI to this would be, but we have some internal tools which would benefit from "get me the changelog for X"-type functionality.

This could be a parallel set of functions to the manifest providers, but the overlap in what it needs to do is almost total. Between this and #66, I'm wondering if manifest_provider functionality could be reimagined as a suite of functions for get_package_xml, get_changelog, etc, possibly backed by functions for stuff like "contents of file from repo", "current state of repo" (with specializations for "supported" hosting providers).

Eg:

def get_package_xml(self):
  return self.impl_.get_file_contents(self.ref_, "package.xml")

def get_changelog(self):
  return self.impl_.get_file_contents(self.ref_, "CHANGELOG.rst")

def get_all_package_xml(self):
  with self.impl_.repo_checkout(self.ref_) as tmpdir:
    for ... in os.walk(tmpdir):
      # look for package XMLs.

Maybe there's an easier way to achieve this?

[dirk-thomas]

That sounds good. The API behind could just take a parameter for a file relative to the package root. Then these two function can just call that single API implemented by each provider.

For a remote-only repository collecting all package.xml files will likely require a clone since listing is not always supported by the remote API.