b4: feature request: send cover as separate e-mail even for a single patch

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

Hello,

first and foremost: b4 is awesome, thank you!

A fixed behaviour of b4 is that when a series to be sent has only one
patch the cover letter is sent in the [PATCH 1/1] mail itself, under
the '---' separator, instead of a separate [PATCH 0/1] mail.

While I'm happy with this policy for simple self-standing patches that
do not need a cover, I find it annoying for other cases, such as:

 * a new iteration of a multi-patch series [0] where all patches except
   one have been applied (or dropped), so the series has become a
   one-patch: keeping the cover letter separate would make it
   consistent with the previous iterations

 * a single patch [1] that is not obvious, so a cover letter is used to
   provide the context, high-level view of a larger topic, etc

[0] https://lore.kernel.org/all/20250528-drm-bridge-convert-to-alloc-api-v4-1-f04e698c9a77@bootlin.com/
[1] https://lore.kernel.org/all/20250618-btf_skip_structs_on_stack-v1-1-e70be639cc53@bootlin.com/

I think both policies should be allowed, under user control. The user
interface to control it should include some of the following, in order
of necessity:

 * a command line flag for 'b4 send' to manually control which policy is
   desired each time a series is sent
 * a per-series parameter, saved within the JSON parameters in the
   cover letter commit, so the policy is consistent across iterations
 * a configuration parameter to set the default policy in lack of the
   two above

I tried some hacks in the b4 code to draft this feature but I just ended
up in breaking other things, so I reached out here for more informed
opinions.

Thanks for considering this addition to b4.

Best regards,
Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com