Merge branch 'da/mergetool-docs'

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