bug#65414: save-some-buffers-functions are unexpectedly called when save-some-buffers pred is non-nil

[Joseph Turner]

Functions which are added to save-some-buffers-functions are always
called when save-some-buffers runs, even when a PRED argument is
specified. This leads to unexpected behavior when PRED is intended to
limit saving buffers to a specific set of file buffers, as in Magit's
magit-save-repository-buffers function.

abbrev--possibly-save is added to save-some-buffers-functions by default
now, so whenever magit-save-repository-buffers runs, Emacs prompts to
save unsaved abbrevs (as well any other functions in the list).

I propose that we pass along the PRED argument of save-some-buffers to
each of save-some-buffers-functions, allowing them to determine what is
appropriate to do. Alternatively, we could pass along the return value
of files--buffers-needing-to-be-saved, which is a list of buffers.

Thoughts?

Joseph