builtin/for-each-ref: factor out core logic into a helper

The implementation of `git for-each-ref` is monolithic within
`cmd_for_each_ref()`, making it impossible to share its logic with other
commands. To enable code reuse for the upcoming `git refs list`
subcommand, refactor the core logic into a shared helper function.

Introduce a new `for-each-ref.h` header to define the public interface
for this shared logic. It contains the declaration for a new helper
function, `for_each_ref_core()`, and a macro for the common usage
options.

Move the option parsing, filtering, and formatting logic from
`cmd_for_each_ref()` into a new helper function named
`for_each_ref_core()`. This helper is made generic by accepting the
command's usage string as a parameter.

The original `cmd_for_each_ref()` is simplified to a thin wrapper that
is only responsible for defining its specific usage array and calling
the shared helper.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

for-each-ref.h

@@ -0,0 +1,26 @@
+#ifndef FOR_EACH_REF_H
+#define FOR_EACH_REF_H
+
+struct repository;
+
+/*
+ * Shared usage string for options common to git-for-each-ref(1)
+ * and git-refs-list(1). The command-specific part (e.g., "git refs list ")
+ * must be prepended by the caller.
+ */
+#define COMMON_USAGE_FOR_EACH_REF \
+	"[--count=<count>] [--shell|--perl|--python|--tcl]\n" \
+	"                         [(--sort=<key>)...] [--format=<format>]\n" \
+	"                         [--include-root-refs] [--points-at=<object>]\n" \
+	"                         [--merged[=<object>]] [--no-merged[=<object>]]\n" \
+	"                         [--contains[=<object>]] [--no-contains[=<object>]]\n" \
+	"                         [(--exclude=<pattern>)...] [--start-after=<marker>]\n" \
+	"                         [ --stdin | <pattern>... ]"
+
+/*
+ * The core logic for for-each-ref and its clones.
+ */
+int for_each_ref_core(int argc, const char **argv, const char *prefix,
+		      struct repository *repo, const char *const *usage);
+
+#endif /* FOR_EACH_REF_H */