Merge branch 'da/mergetool-tool-help'

Merge branch 'da/mergetool-tool-help'

Allow "git mergetool --help" to run outside a Git repository.

* da/mergetool-tool-help:
  difftool: don't assume that default sh is sane
  mergetool: don't require a work tree for --tool-help
  git-sh-setup: move GIT_DIR initialization into a function
  mergetool: use more conservative temporary filenames
  test-lib-functions: adjust style to match CodingGuidelines
  t7610-mergetool: prefer test_config over git config