Merge branch 'rl/am-3way-config'

"git am" learned am.threeWay configuration variable.

* rl/am-3way-config:
git-am: add am.threeWay config variable
t4150-am: refactor am -3 tests
git-am.sh: fix initialization of the threeway variable

diff --cc Documentation/config.txt
Simple merge

diff --cc git-am.sh
Simple merge