Document git-close-current-branch
Document git-close-merged
Document git-dirs
Document git-find-merge
Document git-find-tree
Document git-fugitive
Tweak README to reference `-h` instead of `--help`

It seems `git some-script --help` looks for a manpage even when `git
some-script` is provided by a third-party script. Using `git some-script
-h` instead works.
Document git-rebase-noninteractive
Document git-update-branch
Merge branch 'documentation'

Provide documentation for all commands.