internal/git: add second ref to diffstat
internal/git: add second ref to diffstat

When the user uses --source pointing to a different source branch
compared to the HEAD he's sitting at the diffstat returns the wrong
information, since it's based on HEAD. This patch adds the second
argument to `git diff --stat` to indicate what's the reference it should
use for comparing with the merge base.

Signed-off-by: Bruno Meneguele <82c7731bc2625bcbab88bc571aee6631ba3f66f3@redhat.com>

zaquestion/lab,zaquestion/lab