lib/oe/patch: improve accuracy of patch header extraction

lib/oe/patch: improve accuracy of patch header extraction

When PATCHTOOL = "git", if we need to manually apply a patch and then
commit it (i.e. when git am doesn't work) we try to extract the author /
date / shortlog from the patch header. Make the following improvements
to that extraction process:

* If there's no explicit Subject: but the first line is followed by a
  blank line, isn't an Upstream-Status: or Index: marker and isn't too
  long, then assume it's good enough to be the shortlog. This avoids
  having too many patches with "Upgrade to version x.y" as the shortlog
  (since that is often when patches get added).
* Add --follow to the command we use to find the commit that added the
  patch, so we mostly get the commit that added the patch rather than
  getting stuck on upgrade commits that last moved/renamed the patch
* Populate the date from the commit that added the patch if we were able
  to get the author but not the date from the patch (otherwise you get
  today's date which is less useful).

(From OE-Core rev: 896cfb10ec166a677cbb3b4f8643719cabeb7663)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>