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-index.c

@@ -11,6 +11,7 @@
 static const char diff_cache_usage[] =
 "git diff-index [-m] [--cached] "
 "[<common-diff-options>] <tree-ish> [<path>...]"
+"\n"
 COMMON_DIFF_OPTIONS_HELP;
 
 int cmd_diff_index(int argc, const char **argv, const char *prefix)