commit::print_summary(): don't use format_commit_message()

commit::print_summary(): don't use format_commit_message()

This attempts to fix a regression in git-commit, where non-abbreviated
SHA-1s were printed in the summary.

One possible fix would be to set ctx.abbrev to DEFAULT_ABBREV in the
`if` block, where format_commit_message() is used.

Instead, we do away with the format_commit_message() codeblock
altogether, replacing it with a re-run of log_tree_commit().

We re-run log_tree_commit() with rev.always_show_header set, to force
the invocation of show_log(). The effect of this flag can be seen from
this excerpt from log-tree.c:560, the only area that
rev.always_show_header is checked:

	shown = log_tree_diff(opt, commit, &log);
	if (!shown && opt->loginfo && opt->always_show_header) {
		log.parent = NULL;
		show_log(opt);
		shown = 1;
	}

We also set rev.use_terminator, so that a newline is appended at the end
of the log message. Note that callers in builtin/log.c that also set
rev.always_show_header don't have to set rev.use_terminator, but still
get a newline, because they are wrapped in a pager.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>