How does git track history overwrites?

From: Jens Tröger <jens.troeger@light-speed.de>
Date: May 24, 2026, 23:41 UTC
Message-ID: <089615C1-6526-4ADC-926A-6A232F330DA2@light-speed.de>

Hello,

I’m looking for details and some clarification on a `git fetch` behavior I observed, but can’t quite explain. More context is in this Github comment:

  https://github.com/jenstroeger/python-package-template/pull/1190#discussion_r3288253713

but it boils down to this:

  /tmp/bla > git -c protocol.version=2 fetch origin dda8db18cfc68df532abf33b185ecd12d5b7b326 --depth=1

It seems that sha dda8db1 (tag 1.20.0 previously pointed at it) was replaced due to a suspected history overwrite with fda7769 (tag 1.20.0 now points at it) and git figures that out:

...

  From https://github.com/adamchainz/blacken-docs
  * branch dda8db18cfc68df532abf33b185ecd12d5b7b326 -> FETCH_HEAD

And then:

  /tmp/bla > git checkout FETCH_HEAD
  Note: switching to 'FETCH_HEAD’

...

  HEAD is now at fda7769 Version 1.20.0

And:

  /tmp/bla > cat .git/HEAD 
  fda77690955e9b63c6687d8806bafd56a526e45f
  /tmp/bla > cat .git/FETCH_HEAD 
  dda8db18cfc68df532abf33b185ecd12d5b7b326 'dda8db18cfc68df532abf33b185ecd12d5b7b326' of https://github.com/adamchainz/blacken-docs

I’d like to understand the details some more, and how I could manually make that connection?

Thank you! Jens