built-ins: consistently add "\n" between "usage" and options

Change commands in the "diff" family and "rev-list" to separate the
usage information and option listing with an empty line.

In the case of "git diff -h" we did this already (but let's use a
consistent "\n" pattern there), for the rest these are now consistent
with how the parse_options() API would emit usage.

As we'll see in a subsequent commit this also helps to make the "git
<cmd> -h" output more easily machine-readable, as we can assume that
the usage information is separated from the options by an empty line.

Note that "COMMON_DIFF_OPTIONS_HELP" starts with a "\n", so the
seeming omission of a "\n" here is correct, the second one is provided
by the macro.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff-tree.c

@@ -85,6 +85,7 @@ static int diff_tree_stdin(char *line)
 static const char diff_tree_usage[] =
 "git diff-tree [--stdin] [-m] [-c | --cc] [-s] [-v] [--pretty] [-t] [-r] [--root] "
 "[<common-diff-options>] <tree-ish> [<tree-ish>] [<path>...]\n"
+"\n"
 "  -r            diff recursively\n"
 "  -c            show combined diff for merge commits\n"
 "  --cc          show combined diff for merge commits removing uninteresting hunks\n"