Log message printout cleanups (#3): fix --pretty=oneline

This option is very special, since pretty_print_commit() will _remove_
the newline at the end of it, so we want to have an extra separator
between the things.

I added a honking big comment this time, so that (a) I don't forget this
_again_ (I broke "oneline" several times during this printout cleanup),
and so that people can understand _why_ the code does what it does.

Now, arguably the alternate fix is to always have the '\n' at the end in
pretty-print-commit, but git-rev-list depends on the current behaviour
(but we could have git-rev-list remove it, whatever).

With the big comment, the code hopefully doesn't get broken again. And now
things like

	git log --pretty=oneline --cc --patch-with-stat

works (even if that is admittedly a totally insane combination: if you
want the patch, having the "oneline" log format is just crazy, but hey,
it _works_. Even insane people are people).

Signed-off-by: Junio C Hamano <junkio@cox.net>

log-tree.c

@@ -9,6 +9,7 @@ void show_log(struct rev_info *opt, struct log_info *log, const char *sep)
 	struct commit *commit = log->commit, *parent = log->parent;
 	int abbrev = opt->diffopt.abbrev;
 	int abbrev_commit = opt->abbrev_commit ? opt->abbrev : 40;
+	const char *extra;
 	int len;
 
 	opt->loginfo = NULL;
@@ -18,8 +19,17 @@ void show_log(struct rev_info *opt, struct log_info *log, const char *sep)
 	}
 
 	/*
-	 * Whitespace between commit messages, unless we are oneline
+	 * The "oneline" format has several special cases:
+	 *  - The pretty-printed commit lacks a newline at the end
+	 *    of the buffer, but we do want to make sure that we
+	 *    have a newline there. If the separator isn't already
+	 *    a newline, add an extra one.
+	 *  - unlike other log messages, the one-line format does
+	 *    not have an empty line between entries.
 	 */
+	extra = "";
+	if (*sep != '\n' && opt->commit_format == CMIT_FMT_ONELINE)
+		extra = "\n";
 	if (opt->shown_one && opt->commit_format != CMIT_FMT_ONELINE)
 		putchar('\n');
 	opt->shown_one = 1;
@@ -38,7 +48,7 @@ void show_log(struct rev_info *opt, struct log_info *log, const char *sep)
 	 * And then the pretty-printed message itself
 	 */
 	len = pretty_print_commit(opt->commit_format, commit, ~0u, this_header, sizeof(this_header), abbrev);
-	printf("%s%s", this_header, sep);
+	printf("%s%s%s", this_header, extra, sep);
 }
 
 int log_tree_diff_flush(struct rev_info *opt)