We want to move git-diff-goto-source from rc/tools/git.kak
to rc/filetype/diff.kak (or should we could create
rc/tools/diff.kak?). Either way, create the diff module so we can
formalize this dependency.
Currently this module only provides highlighters, so require it
wherever we reference them.
Keep the diff-select-{file,hunk} commands outside the module because
people might already use them in git buffers.
Git includes them in commit messages and notes, which is sometimes useful
when adding shell script snippets:
# this is included in the commit message!
Also, in rebase buffers, # only marks a comment if it is the first nonblank
character in a line.
The `pick` command in a rebase commit is virtually a no-op, but other
commands actually do things. This commit visually communicates
that fact.
Fixes#3208.
