Build on top of the clean-up done by jk/mergetool and automatically
generate the list of mergetool and difftool backends the build
supports to be included in the documentation.

* da/mergetool-docs:
  doc: generate a list of valid merge tools
  mergetool--lib: list user configured tools in '--tool-help'
  mergetool--lib: add functions for finding available tools
  mergetool--lib: improve the help text in guess_merge_tool()
  mergetool--lib: simplify command expressions