pretty: drop print_email_subject flag

With one exception, the print_email_subject flag is set if and only if
the commit format is email based:

  - in make_cover_letter() we set it along with CMIT_FMT_EMAIL
    explicitly

  - in show_log(), we set it if cmit_fmt_is_mail() is true. That covers
    format-patch as well as "git log --format=email" (or mboxrd).

The one exception is "rev-list --format=email", which somewhat
nonsensically prints the author and date as email headers, but no
subject, like:

  $ git rev-list --format=email HEAD
  commit 64fc4c2cdd4db2645eaabb47aa4bac820b03cdba
  From: Jeff King <peff@peff.net>
  Date: Tue, 19 Mar 2024 19:39:26 -0400

  this is the subject

  this is the body

It's doubtful that this is a useful format at all (the "commit" lines
replace the "From" lines that would make it work as an actual mbox).
But I think that printing the subject as a header (like this patch does)
is the least surprising thing to do.

So let's drop this field, making the code a little simpler and easier to
reason about. Note that we do need to set the "rev" field of the
pretty_print_context in rev-list, since that is used to check for
subject_prefix, etc. It's not possible to set those fields via rev-list,
so we'll always just print "Subject: ". But unless we pass in our
rev_info, fmt_output_email_subject() would segfault trying to figure it
out.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

log-tree.c

@@ -742,7 +742,6 @@ void show_log(struct rev_info *opt)
 		log_write_email_headers(opt, commit, &extra_headers,
 					&ctx.need_8bit_cte, 1);
 		ctx.rev = opt;
-		ctx.print_email_subject = 1;
 	} else if (opt->commit_format != CMIT_FMT_USERFORMAT) {
 		fputs(diff_get_color_opt(&opt->diffopt, DIFF_COMMIT), opt->diffopt.file);
 		if (opt->commit_format != CMIT_FMT_ONELINE)