git: Use merge-tree in patchsets

TOpic: mergetreepatch

revup/git.py

@@ -614,19 +614,28 @@ async def get_diff_summary(
 
     async def merge_tree_commit(
         self,
-        branch1: GitCommitHash,
-        branch2: GitCommitHash,
+        tree1: GitCommitHash,
+        tree2: GitCommitHash,
         new_commit_info: CommitHeader,
-        merge_base: Optional[GitCommitHash] = None,
+        merge_base: GitCommitHash,
+        strategy: Optional[str] = None,
     ) -> GitCommitHash:
         """
         Perform a combined git merge-tree and commit-tree, returning a git commit hash. Raises
         GitConflictException if there are conflicts while merging the trees.
         """
-        args = ["merge-tree", "--write-tree", "--messages", "-z"]
-        if merge_base:
-            args.extend(["--merge-base", merge_base])
-        args.extend([branch1, branch2])
+        args = [
+            "merge-tree",
+            "--write-tree",
+            "--messages",
+            "-z",
+            "--merge-base",
+            merge_base,
+            tree1,
+            tree2,
+        ]
+        if strategy is not None:
+            args.extend(["-X", strategy])
 
         ret, stdout = await self.git(*args, raiseonerror=False)
 
@@ -769,38 +778,14 @@ async def make_virtual_diff_target(
         delete the file, but probably the least confusing of the alternatives of showing no diff and
         showing the old_head->old_base diff.
         """
-        new_index: List[str] = []
-
-        # Transform diff-tree raw output to ls-files style output, taking only the new version
-        for m in RE_RAW_DIFF_TREE_LINE.finditer(
-            await self.git_stdout("diff-tree", "-r", "--no-commit-id", "--raw", old_base, old_head)
-        ):
-            new_index.append(f"{m.group('new_mode')} {m.group('new_hash')} 0\t{m.group('path')}")
-
-        if not new_index:
-            # No files were actually changed, so no diff needs to be applied to new_base
-            return new_base
-
-        temp_index_path = self.get_scratch_dir() + "/index.temp"
-        git_env = {
-            "GIT_INDEX_FILE": temp_index_path,
-        }
-        shutil.copy(f"{self.git_dir}/index", temp_index_path)
-        await self.git("reset", "-q", "--no-refresh", new_base, "--", ":/", env=git_env)
-        await self.git(
-            "update-index",
-            "--index-info",
-            input_str="\n".join(new_index),
-            env=git_env,
-        )
-
-        tree = GitTreeHash(await self.git_stdout("write-tree", env=git_env))
-        new_commit_info = CommitHeader(tree, [parent] if parent else [])
+        new_commit_info = CommitHeader(GitTreeHash(""), [parent] if parent else [])
         new_commit_info.commit_msg = (
-            f"revup virtual diff target\n\n{old_base}\n{old_head}\n{new_base}\n{new_head}"
+            f"revup virtual diff target\n\n{old_base}\n{old_head}\n{new_base}\n{new_head}\n\n"
+            "revup uses this branch internally to generate github viewable diffs. The commits and "
+            "diffs between commits are not meaningful or useful."
         )
 
-        return await self.commit_tree(new_commit_info)
+        return await self.merge_tree_commit(old_head, new_base, new_commit_info, old_base, "ours")
 
     async def soft_reset(self, new_commit: GitCommitHash, env: Dict) -> None:
         await self.git("reset", "--soft", new_commit, env=env)