Reorganize Git for Windows' patches and topic branches (#5406)

This PR introduces a merging-rebase purely to reorganize what is in this
fork's default branch. The main clean-ups are:

- The backports that were needed to apply the security fixes to upstream
Git's `maint-2.40` (which was not maintained very well, failing CI
builds) were dropped; They are already part of v2.48.0 (they were
backports, after all).
- The fall-out of #5389 (I
moved the patches that drop Azure Pipeline/`vcxproj` support to the
beginning, and then dropped all of the patches that modified the
now-no-longer-existing parts).
- I also moved the security fixes to the beginning.
- `fixup!`s were applied.
- Combined many of the `fscache` topics into a single one. Who knows
whether I will upstream this topic at all, it's somewhat incompatible
with partial clones, and FSMonitor might turn out to be good enough on
its own (FSCache would help the cold-cache scenario, though).

All in all, there are still a little under 300 patches (291, to be
precise, down from 319) organized in a little over 100 topic branches
(102, to be precise, down from 121). I have to admit that I am a bit
satisfied by the outcome.

Note that the outcome is strictly tree-same, i.e. this PR's combined
diff is empty.

GitHub complained about a "too long PR body" so I'll post the range-diff
as a follow-up comment.