[Bug report] git format-patch + git am doesn't preserve original commit message

Steps to reproduce:
```
git init
git commit --allow-empty -m 'root'
echo foo > foo
git add .
git commit -m '[tag] foo'
git checkout @~
git format-patch master~..master
cat 0001-tag-foo.patch | git am
```
Expected: commit message generated by `git am` is `[tag] foo`
Actual: commit message generated by `git am` is `foo`

[System Info] git version: git version 2.31.1 cpu: x86_64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh uname: Linux 5.4.98-1-lts #1 SMP Sat, 13 Feb 2021 19:22:14 +0000 x86_64 compiler info: gnuc: 10.2 libc info: glibc: 2.33 $SHELL (typically, interactive shell): /bin/zsh

[Enabled Hooks]