The sleigh repository uses git am to apply a list of patches to a specific Ghidra base commit. These patches are mostly to make Ghidra's decompiler source code reusable as a library and suitable for packaging. You can find more information in sleigh/src/README.md.

Remill has a bunch of additional patches, which improve the semantics themselves (sleigh files). These are applied on top of the sleigh patches and specified in sleigh_ADDITIONAL_PATCHES.

To update the patches or the sleigh base commit, we need to get a cloned Ghidra source tree that only has the sleigh patches applied. From there we will apply the patches in this repository manually and recreate the patch folder.

Go in build/_deps/ghidrasource-src and run git status to make sure you are in a clean state. You might need to run git am --abort to abort the patching process if you had patch failures.
Modify remill's CMakeLists.txt to set sleigh_ADDITIONAL_PATCHES to be empty and re-configure remill. This will apply just the patches of the sleigh project's tag we pinned.
Get the commit hash of the clean Ghidra patches with git rev-parse HEAD and note it as <base-commit>.
Apply the patches in dependencies/sleigh_patches/ one by one with git am ../../../dependencies/sleigh_patches/0001-xyz.patch. If you get any errors, manually apply the patch (you can try git apply ../../../dependencies/sleigh_patches/0001-xyz.patch) and then git add . followed by git am --continue. The goal is to create a commit for every patch.
Delete all the old patches: rm dependencies/sleigh_patches/*.patch.
Recreate the patch list: git format-patch remill-sleigh-7c6b742-base -o ../../../dependencies/sleigh_patches/.
Reconfigure remill's CMake to make sure everything applies correctly and then change sleigh_ADDITIONAL_PATCHES back to include all of the patches in dependencies/sleigh_patches/.

Note: Sometimes you run into issues where git am cannot correctly apply all the patches. This is usually related to whitespace issues. Before exporting the patch list you can run git rebase <base-commit> --whitespace=fix to make sure everything is cleaned up correctly and ready to be applied.