Merge branch 'jc/am-report-3way'

When "git am -3" needs to fall back to an application to a synthesized
preimage followed by a 3-way merge, the paths that needed such treatment
are now reported to the end user, so that the result in them can be
eyeballed with extra care.

* jc/am-report-3way:
  am -3: list the paths that needed 3-way fallback

diff --cc git-am.sh
Simple merge