Merge branch 'ps/pseudo-ref-terminology' into ps/ref-storage-migration

* ps/pseudo-ref-terminology:
  refs: refuse to write pseudorefs
  ref-filter: properly distinuish pseudo and root refs
  refs: pseudorefs are no refs
  refs: classify HEAD as a root ref
  refs: do not check ref existence in `is_root_ref()`
  refs: rename `is_special_ref()` to `is_pseudo_ref()`
  refs: rename `is_pseudoref()` to `is_root_ref()`
  Documentation/glossary: define root refs as refs
  Documentation/glossary: clarify limitations of pseudorefs
  Documentation/glossary: redefine pseudorefs as special refs

refs.h

@@ -1006,8 +1006,52 @@ extern struct ref_namespace_info ref_namespace[NAMESPACE__COUNT];
  */
 void update_ref_namespace(enum ref_namespace namespace, char *ref);
 
-int is_pseudoref(struct ref_store *refs, const char *refname);
-int is_headref(struct ref_store *refs, const char *refname);
+/*
+ * Check whether the provided name names a root reference. This function only
+ * performs a syntactic check.
+ *
+ * A root ref is a reference that lives in the root of the reference hierarchy.
+ * These references must conform to special syntax:
+ *
+ *   - Their name must be all-uppercase or underscores ("_").
+ *
+ *   - Their name must end with "_HEAD". As a special rule, "HEAD" is a root
+ *     ref, as well.
+ *
+ *   - Their name may not contain a slash.
+ *
+ * There is a special set of irregular root refs that exist due to historic
+ * reasons, only. This list shall not be expanded in the future:
+ *
+ *   - AUTO_MERGE
+ *
+ *   - BISECT_EXPECTED_REV
+ *
+ *   - NOTES_MERGE_PARTIAL
+ *
+ *   - NOTES_MERGE_REF
+ *
+ *   - MERGE_AUTOSTASH
+ */
+int is_root_ref(const char *refname);
+
+/*
+ * Pseudorefs are refs that have different semantics compared to
+ * "normal" refs. These refs can thus not be stored in the ref backend,
+ * but must always be accessed via the filesystem. The following refs
+ * are pseudorefs:
+ *
+ * - FETCH_HEAD may contain multiple object IDs, and each one of them
+ *   carries additional metadata like where it came from.
+ *
+ * - MERGE_HEAD may contain multiple object IDs when merging multiple
+ *   heads.
+ *
+ * Reading, writing or deleting references must consistently go either
+ * through the filesystem (pseudorefs) or through the reference
+ * backend (normal ones).
+ */
+int is_pseudo_ref(const char *refname);
 
 /*
  * The following functions have been removed in Git v2.45 in favor of functions