Git-generated trailers are not preserved

Commitizen's `--hook` option truncates the `COMMIT_EDITMSG` file. This wipes the `Signed-off-by` trailer generated by Git's `-s` flag, as well as any other trailers that might have been there before running Commitizen, and forces us to amend our commits after the fact.

We have a temporary workaround in our hook for this, which might offer some insight:

```sh
#!/bin/bash

file="$1"
type="$2"

if [ -z "$type" ]; then # only run on new commits
#
# Save any commit message trailers generated by Git.
#

trailers=$(git interpret-trailers --parse "$file")

#
# Execute the Commitizen hook.
#

(exec < "/dev/tty" && npx --no-install git-cz --hook) || true

#
# Restore any trailers that Commitizen might have overwritten.
#

printf "\n" >> "$file"

while IFS= read -r trailer; do
git interpret-trailers --in-place --trailer "$trailer" "$file"
done <<< "$trailers"
fi
```

Version: cz-cli@4.2.2, cz-conventional-changelog@3.3.0

issue ではソースファイルもテストも指定されていません。まず、`COMMIT_EDITMSG` を書き換える `--hook` の経路を追跡し、Git の `-s` および `interpret-trailers` の動作と比較してください。Git が生成した trailer と既存の trailer が、post-commit の修正を必要とせずに hook を通過すれば完了です。

索引モデルが issue の本文から書いたものです。