Merge branch 'rs/work-around-grep-opt-insanity' into maint

* rs/work-around-grep-opt-insanity:
  Protect scripted Porcelains from GREP_OPTIONS insanity
  mergetool--lib: simplify guess_merge_tool()

diff --cc git-am.sh
Simple merge

diff --cc git-instaweb.sh
Simple merge

diff --cc git-rebase--interactive.sh
Simple merge