Garden-o-matic: Display bugs from the commit BUG=... line in commit summary

This patch also allows multiple bugs on the BUG= line, perhaps having some non-digit characters (see r168027 for an example).

It also occurred to me that with this patch committed, we should require that BUG= (and R=) starts at the beginning of line

NOTRY=true

Review URL: https://codereview.chromium.org/177123013

git-svn-id: svn://svn.chromium.org/blink/trunk@168266 bbb929c8-8fbe-4397-9dbb-9b2b20218538