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

[Nikita Bobko <nikitabobko@xxxxxxxxx>]

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]