rc git: teach "git apply" to work on content, not just diffs

Staging/unstaging/reverting (parts of) the current buffer's file can
be a common use case.

Today "git apply" can do that based on a selection within a diff.
When the selection is on uncommitted content, we can probably assume
that the intent is to use the part of the selection that overlaps
with the +-side of "git diff" (or "git diff --cached" for
"git apply --cached").

Make "git apply" treat selections as content if the buffile is
tracked by Git. This differentiator is not perfect but I don't know
why anyone would want to use the existing "git apply" semantics on
a tracked file. Maybe we should pick a different name.

This feature couples well with "git show-diff", which shows all
lines with unstaged changes (in future it should probably show staged
changes as well).

Whereas on diffs, "git apply" stages the entire hunk if the selection
contains no newline, this does not happen when operating on content.
I didn't yet try implementing that.  I guess the hunks are not as
explicit here.

Closes #5225

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