Add a new and improved mapmerge (#33869)

Key benefits of the new mapmerge include: multi-Z support, effective
reuse of deleted keys, automatic handling of key overflow, and utilizing
a git pre-commit hook to eliminate the need to run batch files manually.

tools/mapmerge2/precommit.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+import os
+import pygit2
+import dmm
+from mapmerge import merge_map
+
+def main(repo):
+    if repo.index.conflicts:
+        print("You need to resolve merge conflicts first.")
+        return 1
+
+    changed = 0
+    for path, status in repo.status().items():
+        if path.endswith(".dmm") and (status & (pygit2.GIT_STATUS_INDEX_MODIFIED | pygit2.GIT_STATUS_INDEX_NEW)):
+            # read the index
+            index_entry = repo.index[path]
+            index_map = dmm.DMM.from_bytes(repo[index_entry.id].read_raw())
+
+            try:
+                head_blob = repo[repo[repo.head.target].tree[path].id]
+            except KeyError:
+                # New map, no entry in HEAD
+                print(f"Converting new map: {path}")
+                assert (status & pygit2.GIT_STATUS_INDEX_NEW)
+                merged_map = index_map
+            else:
+                # Entry in HEAD, merge the index over it
+                print(f"Merging map: {path}")
+                assert not (status & pygit2.GIT_STATUS_INDEX_NEW)
+                head_map = dmm.DMM.from_bytes(head_blob.read_raw())
+                merged_map = merge_map(index_map, head_map)
+
+            # write to the index
+            blob_id = repo.create_blob(merged_map.to_bytes())
+            repo.index.add(pygit2.IndexEntry(path, blob_id, index_entry.mode))
+            changed += 1
+
+            # write to the working directory if that's clean
+            if status & (pygit2.GIT_STATUS_WT_DELETED | pygit2.GIT_STATUS_WT_MODIFIED):
+                print(f"Warning: {path} has unindexed changes, not overwriting them")
+            else:
+                merged_map.to_file(os.path.join(repo.workdir, path))
+
+    if changed:
+        repo.index.write()
+        print(f"Merged {changed} maps.")
+    return 0
+
+if __name__ == '__main__':
+    exit(main(pygit2.Repository(pygit2.discover_repository(os.getcwd()))))