parsemail: Extract building the list of mail references

parsemail: Extract building the list of mail references

We'll need to figure out whether the mail we are parsing is the root of
the thread to automatically build series, and we'll need the list of
references for that.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>