_git: add new configuration options
_git: fix some typos
_git: replace `text` with "text" to make it match the rest of the file
_git: improve a bunch of config option texts, spelling and default values
_git: add missing ':' when using ->bool
_git: change guitool.*.X descriptions to match guitool.X
_git: improve merge.ff config text and default value
_git: add config subsections that take wildcards
_git: git config options: reword TODO to reflect missing merge/diff/filter drivers