[libc++] Add a merge driver that can apply clang-format (#73712)

In preparation for the moment when we'll clang-format the whole code
base, this patch adds a script that can be used to rebase patches across
clang-format changes mechanically, without requiring manual
intervention.

See https://discourse.llvm.org/t/rfc-clang-formatting-all-of-libc-once-and-for-all.

Author: ldionne

libcxx/utils/clang-format-merge-driver.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+
+# This script can be installed in .git/config to allow rebasing old patches across
+# libc++'s clang-format of the whole tree. Most contributors should not require that
+# since they don't have many pre-clang-format patches lying around. This script is to
+# make it easier for contributors that do have such patches.
+#
+# The script is installed by running the following from the root of your repository:
+#
+#   $ git config merge.libcxx-reformat.name "Run clang-format when rebasing libc++ patches"
+#   $ git config merge.libcxx-reformat.driver "libcxx/utils/clang-format-merge-driver.sh %O %A %B %P"
+#
+# This is based on https://github.com/nico/hack/blob/main/notes/auto_git_rebase_across_mechanical_changes.md.
+# Many thanks to Nico Weber for paving the way here.
+
+# Path to the file's contents at the ancestor's version.
+base="$1"
+
+# Path to the file's contents at the current version.
+current="$2"
+
+# Path to the file's contents at the other branch's version (for nonlinear histories, there might be multiple other branches).
+other="$3"
+
+# The path of the file in the repository.
+path="$4"
+
+clang-format --style=file --assume-filename="$path" < "$base" > "$base.tmp"
+mv "$base.tmp" "$base"
+
+clang-format --style=file --assume-filename="$path" < "$current" > "$current.tmp"
+mv "$current.tmp" "$current"
+
+clang-format --style=file --assume-filename="$path" < "$other" > "$other.tmp"
+mv "$other.tmp" "$other"
+
+git merge-file -Lcurrent -Lbase -Lother "$current" "$base" "$other"