history damage in linux.git

[Olaf Hering <olaf@aepfle.de>]

To track the changes in hyperv related files I created some scripts
years ago to automate the process of finding relevant commits in
linux.git. Part of that process is to record the tag when a commit
appeared in mainline. This worked fine, until very recently.

Suddenly years-old commits are declared as having-just-arrived in
linux.git. Look at this example:

  $ git log --oneline -- drivers/input/serio/hyperv-keyboard.c
  2048157 Drivers: hv: vmbus: fix the building warning with hyperv-keyboard
  62238f3 Input: hyperv-keyboard - register as a wakeup source
  c3c4d99 Input: hyperv-keyboard - pass through 0xE1 prefix
  aed06b9 Input: add a driver to support Hyper-V synthetic keyboard
  $ git describe --contains aed06b9
  v4.6-rc1~9^2~792
  $ git show aed06b9 | head
  commit aed06b9cfcabf8644ac5f6f108c0b3d01522f88b
  Author: K. Y. Srinivasan <kys@microsoft.com>
  Date:   Wed Sep 18 12:50:42 2013 -0700

Obviously that and other commits are in the tree since a very long time.

How can I find out whats going on? Is my git(1) 2.8.1 broken, or did
Linus just pull some junk tree (and does he continue to do so)?

Olaf