bug found in `git apply` sub-command

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

1. Clone torvalds/master

2. Checkout a branch at this commit (the commit BEFORE the problematic one to
stage the patch so it would work):
        3e0de271fff77abb933f1b69c213854c3eda9125

3. Get a 'git format-patch' of this commit:
        ccf74128d66ce937876184ad55db2e0276af08d3
        (this is the commit which causes the fault!)

   I did:
      git format-patch 3e0de271fff77a..ccf74128d66ce9
   and it produced
      0001-sched-topology-Assert-non-NUMA-topology-masks-don-t-.patch

4. Try to `git apply` the format-patch generated:
    git apply 0001-sched-topology-Assert-non-NUMA-topology-masks-don-t-.patch

What did you expect to happen? (Expected behavior)

I expected it to apply the patch cleanly. AFAICT, it is a VALID patch which is already committed to Linux.

What happened instead? (Actual behavior)

It gives me the error message:

error: patch fragment without header at line 58: @@ -465,6 +465,9 @@ void update_siblings_masks(unsigned int cpuid)

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

It shouldn't raise an error on applying this patch; its complaining about a patch-fragment _inside_ the message.

Anything else you want to add:

My curiosity was raised since simply doing:

   git am 0001-sched-topology-Assert-non-NUMA-topology-masks-don-t-.patch

applies _exactly_ the same patch without any guff.

I would _really_ expect `git am` and `git apply` to work in exactly the same way, except for the expected difference of the actual commit. The "patching" part should work identically.

I tested some other behaviours and `git cherry-pick` of the offending commit works fine. (Although, I suspect that this does not actually "parse" the commit message, so probably not and apples-to-apples comparison.)

I looked at this patch and in the "message" section, it contains a patch- fragment. Moreover, that patch-fragment is left-justified.

A simple fix to avoid this problem is indenting the message-embedded patch- fragment by a couple spaces.

It appears that `git apply`, as it is parsing the message triggers on the patch- fragment INSIDE the message section. Comparatively, `git am` does NOT trigger and both patches and commits it without issue.

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 2.30.0 cpu: x86_64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh uname: Linux 5.10.15-172.current #1 SMP PREEMPT Fri Feb 12 16:35:57 UTC 2021 x86_64 compiler info: gnuc: 9.3 libc info: glibc: 2.31 $SHELL (typically, interactive shell): /bin/bash

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