Merge branch 'so/separate-field-for-m-and-diff-merges'

Internal API clean-up to handle two options "diff-index" and "log"
have, which happen to share the same short form, more sensibly.

* so/separate-field-for-m-and-diff-merges:
  revision: add separate field for "-m" of "diff-index -m"

diff --cc revision.c
Simple merge