While not immediately obvious, git-show-ref(1) actually implements three
different subcommands:

    - `git show-ref <patterns>` can be used to list references that
      match a specific pattern.

    - `git show-ref --verify <refs>` can be used to list references.
      These are _not_ patterns.

    - `git show-ref --exclude-existing` can be used as a filter that
      reads references from standard input, performing some conversions
      on each of them.

Let's make this more explicit in the code by splitting up the three
subcommands into separate functions. This also allows us to address the
confusingly named `patterns` variable, which may hold either patterns or
reference names.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 builtin/show-ref.c | 100 ++++++++++++++++++++++++---------------------
 1 file changed, 53 insertions(+), 47 deletions(-)

diff --git a/builtin/show-ref.c b/builtin/show-ref.c
index 7efab14b96c..56ee3250c5f 100644
--- a/builtin/show-ref.c
+++ b/builtin/show-ref.c
@@ -104,7 +104,7 @@ static int add_existing(const char *refname,
  * (4) ignore if refname is a ref that exists in the local repository;
  * (5) otherwise output the line.
  */
-static int exclude_existing(const char *match)
+static int cmd_show_ref__exclude_existing(const char *match)
 {
 	static struct string_list existing_refs = STRING_LIST_INIT_DUP;
 	char buf[1024];
@@ -142,6 +142,53 @@ static int exclude_existing(const char *match)
 	return 0;
 }
 
+static int cmd_show_ref__verify(const char **refs)
+{
+	if (!refs || !*refs)
+		die("--verify requires a reference");
+
+	while (*refs) {
+		struct object_id oid;
+
+		if ((starts_with(*refs, "refs/") || !strcmp(*refs, "HEAD")) &&
+		    !read_ref(*refs, &oid)) {
+			show_one(*refs, &oid);
+		}
+		else if (!quiet)
+			die("'%s' - not a valid ref", *refs);
+		else
+			return 1;
+		refs++;
+	}
+
+	return 0;
+}
+
+static int cmd_show_ref__patterns(const char **patterns)
+{
+	struct show_ref_data show_ref_data = {
+		.patterns = (patterns && *patterns) ? patterns : NULL,
+	};
+
+	if (show_head)
+		head_ref(show_ref, &show_ref_data);
+	if (heads_only || tags_only) {
+		if (heads_only)
+			for_each_fullref_in("refs/heads/", show_ref, &show_ref_data);
+		if (tags_only)
+			for_each_fullref_in("refs/tags/", show_ref, &show_ref_data);
+	} else {
+		for_each_ref(show_ref, &show_ref_data);
+	}
+	if (!found_match) {
+		if (verify && !quiet)
+			die("No match");
+		return 1;
+	}
+
+	return 0;
+}
+
 static int hash_callback(const struct option *opt, const char *arg, int unset)
 {
 	hash_only = 1;
@@ -185,56 +232,15 @@ static const struct option show_ref_options[] = {
 
 int cmd_show_ref(int argc, const char **argv, const char *prefix)
 {
-	struct show_ref_data show_ref_data = {0};
-	const char **patterns;
-
 	git_config(git_default_config, NULL);
 
 	argc = parse_options(argc, argv, prefix, show_ref_options,
 			     show_ref_usage, 0);
 
 	if (exclude_arg)
-		return exclude_existing(exclude_existing_arg);
-
-	patterns = argv;
-	if (!*patterns)
-		patterns = NULL;
-
-	if (verify) {
-		if (!patterns)
-			die("--verify requires a reference");
-		while (*patterns) {
-			struct object_id oid;
-
-			if ((starts_with(*patterns, "refs/") || !strcmp(*patterns, "HEAD")) &&
-			    !read_ref(*patterns, &oid)) {
-				show_one(*patterns, &oid);
-			}
-			else if (!quiet)
-				die("'%s' - not a valid ref", *patterns);
-			else
-				return 1;
-			patterns++;
-		}
-		return 0;
-	}
-
-	show_ref_data.patterns = patterns;
-
-	if (show_head)
-		head_ref(show_ref, &show_ref_data);
-	if (heads_only || tags_only) {
-		if (heads_only)
-			for_each_fullref_in("refs/heads/", show_ref, &show_ref_data);
-		if (tags_only)
-			for_each_fullref_in("refs/tags/", show_ref, &show_ref_data);
-	} else {
-		for_each_ref(show_ref, &show_ref_data);
-	}
-	if (!found_match) {
-		if (verify && !quiet)
-			die("No match");
-		return 1;
-	}
-	return 0;
+		return cmd_show_ref__exclude_existing(exclude_existing_arg);
+	else if (verify)
+		return cmd_show_ref__verify(argv);
+	else
+		return cmd_show_ref__patterns(argv);
 }
-- 
2.42.0