Bug 1822349 - monorepo-migration: add convert-to-hg script

This script rewrites the git repo's history and converts it to
mercurial, but stops short of merging it with mozilla-central for now.

The rewrite steps are:
- replacing octopus merges with a series of 2-parent merges, since
  mercurial doesn't have octopus merges, appending " (part N)" to the
  first line of the commit message for each
- moving all files from the repository root to `mobile/android/`
- replacing references to pull request numbers in commit messages with
  their full URL

Git-cinnabar is used for the conversion to mercurial, with a custom
change to better detect file copies/moves.

monorepo-migration/convert-to-hg.sh

@@ -0,0 +1,97 @@
+#!/bin/sh
+
+set -e
+set -x
+
+# FIXME: make rename detection configurable and switch to glandium/git-cinnabar:master
+CINNABAR=https://github.com/jcristau/git-cinnabar
+CINNABAR_BRANCH=detect-more-renames
+
+SOURCE_REPO=https://github.com/mozilla-mobile/firefox-android
+SOURCE_REPO_NAME=${SOURCE_REPO##*/}
+SOURCE_BRANCH=main
+
+DEST_REPO="$(readlink -f "$1")"
+DEST_SUBDIR=mobile/android
+
+EXPRESSIONS_FILE_PATH="$(dirname $(readlink -f "$0"))/data/message-expressions.txt"
+
+TEMPDIR=$(mktemp -d)
+trap "rm -rf ${TEMPDIR}" EXIT
+CINNABAR_PATH="${TEMPDIR}/git-cinnabar"
+SOURCE_REPO_PATH="${TEMPDIR}/${SOURCE_REPO_NAME}"
+
+setup_temporary_repo() {
+  git clone "${SOURCE_REPO}" "${SOURCE_REPO_PATH}"
+  git -C "${SOURCE_REPO_PATH}" config user.email=release@mozilla.com
+  git -C "${SOURCe_REPO_PATH}" config user.name="Mozilla Release Engineering"
+}
+
+setup_cinnabar() {
+  git clone --branch "${CINNABAR_BRANCH}" --depth 1 "${CINNABAR}" "${CINNABAR_PATH}"
+  make -C "${CINNABAR_PATH}"
+}
+
+rewrite_git_history() {
+  # replace octopus merges with a series of two-parent merges
+  # commit-filter is called with args "<tree> [-p <parent>]*" and with the commit message on stdin
+  FILTER_BRANCH_SQUELCH_WARNING=1 \
+  git -C "${SOURCE_REPO_PATH}" filter-branch \
+    --force \
+    --commit-filter \
+      'if test "$#" -lt 7; then
+         # 0, 1 or 2 parents, call commit-tree as-is
+         git commit-tree "$@"
+       else
+         # octopus merge, replace it with a series of manual merges
+         commit_message=$(mktemp)
+         cat > $commit_message
+
+         final_tree="$1"
+         shift
+         shift
+         p1="$1"
+         shift
+         part=1
+         while test "$#" -ge 2; do
+           shift
+           p2="$1"
+           shift
+           tree=$(git merge-tree --write-tree --allow-unrelated-histories "$p1" "$p2")
+           p1="$(sed "1s/$/ (part $part)/" $commit_message | git commit-tree "$tree" -p "$p1" -p "$p2")"
+           part=$((part + 1))
+         done
+         rm -f $commit_message
+         # sanity check that the end result is what we expected
+         git diff-tree --exit-code $tree $final_tree
+         echo "$p1"
+     fi' \
+        -- "$SOURCE_BRANCH"
+  # to-subdirectory-filter: move everything to $DEST_SUBDIR
+  # replace-message: replace references to pull requests with the full URL
+  # preserve-commit-hashes: because the filter-branch step also changes
+  #                         hashes, as does the conversion to hg, rewriting
+  #                         hashes here would only point at temporary commit
+  #                         objects which wouldn't help
+  git -C "${SOURCE_REPO_PATH}" filter-repo \
+    --force \
+    --to-subdirectory-filter "$DEST_SUBDIR/" \
+    --replace-message "$EXPRESSIONS_FILE_PATH" \
+    --preserve-commit-hashes \
+    --refs "$SOURCE_BRANCH"
+}
+
+convert_to_hg() {
+  PATH="${PATH}:${CINNABAR_PATH}" \
+  git -C "${SOURCE_REPO_PATH}" \
+      -c cinnabar.experiments=merge \
+      push "hg::${DEST_REPO}" "${SOURCE_BRANCH}"
+}
+
+setup_cinnabar
+setup_temporary_repo
+rewrite_git_history
+if ! [ -d "${DEST_REPO}" ]; then
+  hg init "${DEST_REPO}"
+fi
+convert_to_hg