show: skip diff when possible

Running:

  git show -s $commit

will still compute a diff for $commit, even though we aren't going to
show it. This is wasted computation, since it cannot affect the output
or exit code of the program.

In the more general case:

  - if the requested diff format is NO_OUTPUT, then we won't change the
    output of the diff itself

  - if rev_info.always_show_header is set, then we will show the commit
    regardless of whether the diff is empty (which is true for git-show,
    for example, but not git-log)

  - we don't use --exit-code here (should check?)

Author: peff

log-tree.c

@@ -1105,6 +1105,10 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 	if (!all_need_diff && !opt->merges_need_diff)
 		return 0;
 
+	if (opt->diffopt.output_format == DIFF_FORMAT_NO_OUTPUT &&
+	    opt->always_show_header)
+		return 0;
+
 	parse_commit_or_die(commit);
 	oid = get_commit_tree_oid(commit);