[gp_release] Support the [summary] format in git log

Signed-off-by: David Greaves <david.greaves@jollamobile.com>
Correct validation of pkg_dir

Signed-off-by: David Greaves <david.greaves@jollamobile.com>
[gp_release] PKG_DIR and PKG_FILE can specify rpm/ dir and which .spec/yaml to use.
Improve .changes handling

Signed-off-by: David Greaves <david.greaves@jollamobile.com>