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