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

[Nikita Bobko <nikitabobko@gmail.com>]

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


[Enabled Hooks]