https://swiftshader-review.googlesource.com/c/SwiftShader/+/50749
squashed together the changes from two `git subtree pull` commands (for
SPIRV-Headers and SPIRV-Tools), which resulted in a change with a single
parent.

`git subtree` expects a merge commit with a single set of `git-subtree-*`
metadata tags. The squash resulted in a change description with two sets
of these, confusing `git subtree` even further.

The process to fix this is probably not the easiest, but the first one I found that works:

  # Rollback to the parent change of 50749
  git checkout 43bb60e1
  # Perform the subtree pull again for SPIRV-Headers (to 104ecc356)
  git subtree pull --prefix third_party/SPIRV-Headers https://github.com/KhronosGroup/SPIRV-Headers 104ecc356c1bea4476320faca64440cd1df655a3 --squash -m "Update SPIR-V Headers"
  # Note the second parent of the merge for the SPIRV-Headers (Merge: 43bb60e1 ded361b4)
  git show -1
  # Perform the subtree pull again for SPIRV-Tools (to b0e22d28f)
  git subtree pull --prefix third_party/SPIRV-Tools https://github.com/KhronosGroup/SPIRV-Tools b0e22d28f5ec891d6b256703575d08fce4228bd9 --squash -m "Update SPIR-V Tools"
  # Note the second parent of the merge for the SPIRV-Tools (Merge: a00ce432a9d bdb9eea9)
  git show -1
  # Jump back to master
  git checkout origin/master
  # Revert 50749
  git revert 9dff6a3b
  # Re-merge the squashed update change for SPIRV-Headers
  git merge -Xsubtree="third_party/SPIRV-Headers" ded361b4
  # Re-merge the squashed update change for SPIRV-Tools
  git merge -Xsubtree="third_party/SPIRV-Tools" bdb9eea9

Bug: b/174239232
Change-Id: I0b498184493f0fcb00b1684182b6a89076a5cca0