sendemail.maximumDiffSize: Limit the size of diffs sent by email

If a unified diff included in an email will exceed the limit
configured by the system administrator, only the affected file
paths are listed in the email instead. This gives interested parties
some context on the size and scope of the change, without killing
their inbox.

Change-Id: I93c6068e9fd04095b6ad353b736126b29c4ed4fc
3 files changed