[BUG REPORT] `git rebase --exec` shouldn't run the exec command when there is nothing to rebase

Steps: git rebase HEAD --exec "echo foo"

EXPECTED: since 0 commits are going to be rebased then I expect "foo"
NOT to be printed
ACTUAL:   "foo" is printed

A real-world example where such behavior is really unexpected - I have a script that signs a series of commits like this: ```bash head="$(git rev-parse HEAD)"

# Generate some commits (in my case it's cherry-picking commits from public GitHub subtree repo to internal monorepo)

file="$(mktemp)" git rebase $head --exec "git log -1 --pretty='%B' > $file" \ --exec "echo 'closes 'https://github.com/JetBrains/intellij-community/pull/$1' >> $file" \ --exec "git commit --amend -F $file" ``` But if none of the commits were generated then `git rebase --exec` will amend the HEAD which is not expected. It means that I have to process the case when 0 zero commits are generated separately.

If you agree that it's a bug then, most likely, it won't be possible to fix it because it would break compatibility. Well, yes then this bug report is JFYI.

[System Info] git version: git version 2.33.0 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: 11.1 libc info: glibc: 2.33 $SHELL (typically, interactive shell): /bin/zsh

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