Suggestion for b4 prep: new flag to collect email addresses on a per patch basis

[Brian Masney <bmasney@redhat.com>]

Hi,

I recently had to send out this 114 patch series that covers changes to
drivers/clk to migrate away from a deprecated API:

https://lore.kernel.org/linux-clk/20250811-clk-for-stephen-round-rate-v1-0-b3bf97b038dc@redhat.com/

40 of these clk drivers have entry in the MAINTAINERS file, so
'b4 prep --auto-to-cc' adds everyone to the To line for the whole
series. Most people only have 1 or 2 drivers that's relevant to them
in the big series, so it creates a lot of noise for people.

For big series like this, what do you think about adding an option to
'b4 prep' to have it only put someone on the To line for the patch(es)
of interest? So for example, let's say we have two patches that make
changes to drivers/clk/driver1.c and drivers/clk/driver2.c.

- [PATCH 0/2] clk: some changes  # Sent out to everyone. Union of all
                                 # email addresses below.
- [PATCH 1/2] clk: driver1:      # Only sent to driver1 maintainers
- [PATCH 2/2] clk: driver2:      # Only sent to driver2 maintainers

I could have sent out maybe 80 ish or so small patch series, but there's
downsides to that as well for the maintainer.

Brian