[BUG] git add -A --dry-run adds files to .git directory

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Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)

mat@charly:~$ git init repo
mat@charly:~$ cat /dev/random | head -c 100000000 > repo/randomfile
mat@charly:~$ git -C repo add -A -n
mat@charly:~$ du -hs repo/.git

What did you expect to happen? (Expected behavior)

  76K    repo/.git

What happened instead? (Actual behavior)

  97M    repo/.git

What's different between what you expected and what actually happened?

Even though I specified -n on the command line to make a dry run, it 
still added the large file as an object.

Anything else you want to add:

This is counter-intuitive behaviour.  When the documentation says "Don't 
actually add the file(s)", it's reasonable to expect that this command 
will leave the repository state as it found it.  If there's a good 
reason why these files should be added to the .git directory, that 
behaviour should be made clear in the documentation, probably including 
the reason why that counter-intuitive behaviour is necessary.

This bit me due to a script that was using git add -A -n to detect if a 
repository had been locally modified.  While I do have alternative 
(actually better) ways to get this information, it wasn't an 
unreasonable command to use in the first place, and it caused a real 
issue in a production system.

Please review the rest of the bug report below.
You can delete any lines you don't wish to share.


[System Info]
git version:
git version 2.30.1 (Apple Git-130)
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Darwin 20.6.0 Darwin Kernel Version 20.6.0: Mon Aug 30 06:12:21 
PDT 2021; root:xnu-7195.141.6~3/RELEASE_X86_64 x86_64
compiler info: clang: 13.0.0 (clang-1300.0.29.3)
libc info: no libc information available
$SHELL (typically, interactive shell): /bin/bash


[Enabled Hooks]