parse-options.[ch]: consistently use "enum parse_opt_flags"

Use the "enum parse_opt_flags" instead of an "int flags" as arguments
to the various functions in parse-options.c.

Even though this is an enum bitfield there's there's a benefit to
doing this when it comes to the wider C ecosystem. E.g. the GNU
debugger (gdb) will helpfully detect and print out meaningful enum
labels in this case. Here's the output before and after when breaking
in "parse_options()" after invoking "git stash show":

Before:

    (gdb) p flags
    $1 = 9

After:

    (gdb) p flags
    $1 = (PARSE_OPT_KEEP_DASHDASH | PARSE_OPT_KEEP_UNKNOWN)

Of course as noted in[1] there's a limit to this smartness,
i.e. manually setting it with unrelated enum labels won't be
caught. There are some third-party extensions to do more exhaustive
checking[2], perhaps we'll be able to make use of them sooner than
later.

We've also got prior art using this pattern in the codebase. See
e.g. "enum bloom_filter_computed" added in 312cff5 (bloom: split
'get_bloom_filter()' in two, 2020-09-16) and the "permitted" enum
added in ce91028 (add -p: fix checking of user input, 2020-08-17).

1. https://lore.kernel.org/git/87mtnvvj3c.fsf@evledraar.gmail.com/
2. https://github.com/sinelaw/elfs-clang-plugins/blob/master/enums_conversion/README.md

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

Author: avar

parse-options.c

@@ -481,7 +481,8 @@ static void parse_options_check(const struct option *opts)
 
 static void parse_options_start_1(struct parse_opt_ctx_t *ctx,
 				  int argc, const char **argv, const char *prefix,
-				  const struct option *options, int flags)
+				  const struct option *options,
+				  enum parse_opt_flags flags)
 {
 	ctx->argc = argc;
 	ctx->argv = argv;
@@ -506,7 +507,8 @@ static void parse_options_start_1(struct parse_opt_ctx_t *ctx,
 
 void parse_options_start(struct parse_opt_ctx_t *ctx,
 			 int argc, const char **argv, const char *prefix,
-			 const struct option *options, int flags)
+			 const struct option *options,
+			 enum parse_opt_flags flags)
 {
 	memset(ctx, 0, sizeof(*ctx));
 	parse_options_start_1(ctx, argc, argv, prefix, options, flags);
@@ -838,8 +840,9 @@ int parse_options_end(struct parse_opt_ctx_t *ctx)
 }
 
 int parse_options(int argc, const char **argv, const char *prefix,
-		  const struct option *options, const char * const usagestr[],
-		  int flags)
+		  const struct option *options,
+		  const char * const usagestr[],
+		  enum parse_opt_flags flags)
 {
 	struct parse_opt_ctx_t ctx;
 	struct option *real_options;

parse-options.h

@@ -213,7 +213,8 @@ struct option {
  */
 int parse_options(int argc, const char **argv, const char *prefix,
 		  const struct option *options,
-		  const char * const usagestr[], int flags);
+		  const char * const usagestr[],
+		  enum parse_opt_flags flags);
 
 NORETURN void usage_with_options(const char * const *usagestr,
 				 const struct option *options);
@@ -262,15 +263,16 @@ struct parse_opt_ctx_t {
 	const char **out;
 	int argc, cpidx, total;
 	const char *opt;
-	int flags;
+	enum parse_opt_flags flags;
 	const char *prefix;
 	const char **alias_groups; /* must be in groups of 3 elements! */
 	struct option *updated_options;
 };
 
 void parse_options_start(struct parse_opt_ctx_t *ctx,
 			 int argc, const char **argv, const char *prefix,
-			 const struct option *options, int flags);
+			 const struct option *options,
+			 enum parse_opt_flags flags);
 
 int parse_options_step(struct parse_opt_ctx_t *ctx,
 		       const struct option *options,