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-files.c

@@ -15,6 +15,7 @@
 
 static const char diff_files_usage[] =
 "git diff-files [-q] [-0 | -1 | -2 | -3 | -c | --cc] [<common-diff-options>] [<path>...]"
+"\n"
 COMMON_DIFF_OPTIONS_HELP;
 
 int cmd_diff_files(int argc, const char **argv, const char *prefix)

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)

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"

builtin/diff.c

@@ -30,7 +30,8 @@ static const char builtin_diff_usage[] =
 "   or: git diff [<options>] [--merge-base] <commit> [<commit>...] <commit> [--] [<path>...]\n"
 "   or: git diff [<options>] <commit>...<commit> [--] [<path>...]\n"
 "   or: git diff [<options>] <blob> <blob>\n"
-"   or: git diff [<options>] --no-index [--] <path> <path>\n"
+"   or: git diff [<options>] --no-index [--] <path> <path>"
+"\n"
 COMMON_DIFF_OPTIONS_HELP;
 
 static const char *blob_path(struct object_array_entry *entry)

builtin/rev-list.c

@@ -21,6 +21,7 @@
 
 static const char rev_list_usage[] =
 "git rev-list [<options>] <commit-id>... [-- <path>...]\n"
+"\n"
 "  limiting output:\n"
 "    --max-count=<n>\n"
 "    --max-age=<epoch>\n"