Makefile: add changelog target

Maintaining a changelog for each new version or release of Podman helps
users to quickly skim for new changes.  Add a `make changelog` target to
facilitate creating a new log.

There are two env variables to control the base and target commit for
the new log.  The output gets prepended to the changelog.txt file, which
is a textfile in following format:
- Changelog for $(CHANGELOG_TARGET) (ISO-8601 DATE):
  * Commit subject
  * Commit subject...

Notice that the list of commit subjects excludes merge commits, and can
be manually modified after generation if needed.

`CHANGELOG_BASE=v0.3.2 CHANGELOG_TARGET=v0.3.3 make changelog` would
generate the following shortened output to the changelog.txt file:

Changelog for v0.3.3 (2018-03-17):
  * Bump to v0.3.3
  * Fix build after c/image changes
  * Update containers/image
  * Fix E2E tests
  * Address review comments
  * Fix E2E tests
  * Add restart to main podman manpage
  * Add podman restart to podman bash completions and commands

Signed-off-by: Valentin Rothberg <vrothberg@suse.com>

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