read-tree: migrate to parse-options

Cleanup the documentation to explicitly state that --exclude-directory
is only meaningful when used with -u. Also make the documentation more
consistent with the usage message printed with read-tree --help-all.

The -m, --prefix, --reset options are performing similar actions
(setting some flags, read_cache_unmerged(), checking for illegal option
combinations). Instead of performing these actions when the options are
parsed, we delay performing them until after parse-opts has finished.

The bit fields in struct unpack_trees_options have been promoted to full
unsigned ints. This is necessary to avoid "foo ? 1 : 0" constructs to
set these fields.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.h

@@ -17,18 +17,18 @@ struct unpack_trees_error_msgs {
 };
 
 struct unpack_trees_options {
-	unsigned int reset:1,
-		     merge:1,
-		     update:1,
-		     index_only:1,
-		     nontrivial_merge:1,
-		     trivial_merges_only:1,
-		     verbose_update:1,
-		     aggressive:1,
-		     skip_unmerged:1,
-		     initial_checkout:1,
-		     diff_index_cached:1,
-		     gently:1;
+	unsigned int reset,
+		     merge,
+		     update,
+		     index_only,
+		     nontrivial_merge,
+		     trivial_merges_only,
+		     verbose_update,
+		     aggressive,
+		     skip_unmerged,
+		     initial_checkout,
+		     diff_index_cached,
+		     gently;
 	const char *prefix;
 	int pos;
 	struct dir_struct *dir;