Collect a set of repositories, each in its own subdir, into one mono-repo.

merge-repos.sh

#!/bin/bash
# Merges a set of repositories, the “sources”, formerly hosted on “from”
# into subdirectories of one repository
# meant to be moved to a new repository hosting provider.
#
# git version: 2.24.0
# Mark Kubacki, 2020-01-08

if (( $# < 1 )); then
  >&2 printf "Usage: $0 [repo | repo…]\n"
  exit 2
fi

set -euop pipefail

# Change this to your Gitlab, if need be. The 'slug' below will be %s.
: ${hosting:="https://source.developers.google.com/p/${USER}/r/%s"}
# This is the slug for the destination repository.
: ${D:="unrelated-containers"}
# I'm moving away from Github.
: ${leave:="[email protected]:${USER}/%s.git"}

declare -a sources=()

workdir="$(mktemp -d -t merge-repos.XXXXXX)"
cd "${workdir}"
trap "rm -rf '${workdir}'" EXIT

# Fetch all relevant repositories.
>&2 printf "==== git clone …\n"
for source; do
  if [[ ! -d "${source}" ]]; then
    printf -v from "${leave}" "${source}"
    if ! git clone "${from}"; then
      >&2 printf "ERR, skipping: %s ← %s\n" "${source}" "${from}"
      continue
    fi
  fi

  sources+=("${source}")
done

# Go through every source repository and move its contents one directory level down.
>&2 printf "==== push to subdirectories\n"
for source in "${sources[@]}"; do
  cd "${source}"
  FILTER_BRANCH_SQUELCH_WARNING=1 \
  git filter-branch -f --prune-empty --tree-filter '
    mkdir -p "'${source}'"
    git ls-tree --name-only $GIT_COMMIT | xargs -I files mv files "'${source}'"
  '
  cd ..
done

# Pull everything into the super-repository.
>&2 printf "==== assemble the mono-repository ====\n"
printf -v destination "${hosting}" "${D}"

if git clone -o 'destination' "${destination}"; then
  cd "${D}"
else
  mkdir "${D}"
  cd $_
  git init .
  git commit --allow-empty -m "Create this mono-repository"
  git remote add 'destination' "${destination}"

  #gcloud source repos create ${D}
fi

for source in "${sources[@]}"; do
  if [[ -d "${source}" ]]; then
    >&2 printf "EXISTS, skipping: %s\n" "${source}"
    continue
  fi

  git remote add -f "${source}" ../"${source}"
  git merge -s ours --allow-unrelated-histories --no-commit "${source}/master"
  # No need to work with another prefix: The files already are in distinct subdirectories.
  git read-tree --prefix=""/ -u "${source}/master"
  git commit -m "subtree-merge: ${source}"
done

git push destination master
cd ..

# fin
exit 0

If you already have a super-repository or “target mono-repo”, you could do the following. For the sake of this example I'll call the destination directory cmd/indexer. Its git checkout is in strayrepo/:

cd strayrepo/
FILTER_BRANCH_SQUELCH_WARNING=1 \
git filter-branch -f --prune-empty --tree-filter '
 mkdir -p "cmd/indexer"
 git ls-tree --name-only $GIT_COMMIT | xargs -I files mv files "cmd/indexer/"
'

cd monorepo
git remote add -f "cmd-indexer" ../strayrepo
git merge -s ours --allow-unrelated-histories --no-commit "cmd-indexer/master"
git read-tree --prefix=/ -u "cmd-indexer/master"
git commit -m "cmd/indexer: Package from subtree-merge"

If the source repository already contains a directory you want to move it entirely to, or below, — for example a cmd/ but everything shall go into cmd/indexer/… —, do this:

cd strayrepo/

FILTER_BRANCH_SQUELCH_WARNING=1 \
git filter-branch -f --prune-empty --tree-filter '
  mkdir -p "xmd/indexer"
  git ls-tree --name-only $GIT_COMMIT | xargs -I files mv files "xmd/indexer"
'

FILTER_BRANCH_SQUELCH_WARNING=1 \
git filter-branch --tree-filter '
  mv xmd cmd
' --force HEAD