[ruby/digest] Adjust styles [ci skip]

- Use the C90 standard style for comments, since this gem supports
  versions prior to ruby 2.7.

- Adjust the indentation.

ruby/digest@4751402e50

Author: nobu

ext/digest/digest.h

@@ -77,16 +77,16 @@ static inline VALUE
 rb_digest_make_metadata(const rb_digest_metadata_t *meta)
 {
 #if EXTSTATIC
-  // The extension is built as a static library, so safe to refer to
-  // rb_digest_wrap_metadata directly.
-  extern VALUE rb_digest_wrap_metadata(const rb_digest_metadata_t *meta);
-  return rb_digest_wrap_metadata(meta);
+    /* The extension is built as a static library, so safe to refer to
+     * rb_digest_wrap_metadata directly. */
+    extern VALUE rb_digest_wrap_metadata(const rb_digest_metadata_t *meta);
+    return rb_digest_wrap_metadata(meta);
 #else
-  // The extension is built as a shared library, so we can't refer to
-  // rb_digest_wrap_metadata directly.
+    /* The extension is built as a shared library, so we can't refer
+     * to rb_digest_wrap_metadata directly. */
 # ifdef DIGEST_USE_RB_EXT_RESOLVE_SYMBOL
-    // If rb_ext_resolve_symbol() is available, use it to get the address of
-    // rb_digest_wrap_metadata.
+    /* If rb_ext_resolve_symbol() is available, use it to get the address of
+     * rb_digest_wrap_metadata. */
     typedef VALUE (*wrapper_func_type)(const rb_digest_metadata_t *meta);
     static wrapper_func_type wrapper;
     if (!wrapper) {
@@ -96,7 +96,8 @@ rb_digest_make_metadata(const rb_digest_metadata_t *meta)
     }
     return wrapper(meta);
 # else
-    // If rb_ext_resolve_symbol() is not available, keep using untyped data.
+    /* If rb_ext_resolve_symbol() is not available, keep using untyped
+     * data. */
 # undef RUBY_UNTYPED_DATA_WARNING
 # define RUBY_UNTYPED_DATA_WARNING 0
     return rb_obj_freeze(Data_Wrap_Struct(0, 0, 0, (void *)meta));