Start the merging-rebase to v2.20.1

In preparation for rebasing onto v2.21.0-rc, this janitorial
merging-rebase juggles a few things around and updates others:

- The `case-insensitive-abspath`, `unc-path-w-backslashes`,
  `azure-pipelines` and `stash-in-c` branches were updated to reflect
  the versions that made it into v2.21.0-rc0.

- As the patch in `kraai/2006` was folded into `stash-in-c` upstream, we
  did the same here.

- The branches `status-no-lock-index` and
  `phase-out-show-ignored-directory-gracefully`, intended to help with
  phasing out two Git for Windows-only features (that made it into core
  Git in different forms), were moved to the bottom of the branch
  thicket.

- The `funny-cased-cwd`, `remove-ipv6-fallback`, `open-in-gdb`,
  `mingw-build-options-fix`, `http-version`, `builtin-rebase-perf-fix`,
  `test-oid-fix-windows` and `external-diff-save-env`
  patches were moved into the `ready-for-upstream` thicket.

- The `rebase-post-checkout-hook` and the `builtin-rebase--am` branch
  was updated to the version that made it into v2.21.0-rc0, and moved to
  the `ready-for-upstream` thicket.

- The FSCache-related topics were moved closer together.

- The commit message of `MinGW: Use MakeMaker to build the Perl
  libraries` was changed to reflect the fact that we do not even use
  MakeMaker anymore.

- The patch `Tests: optionally skip redirecting stdin/stdout/stderr` was
  dropped, as the `debug` function learned to un-redirect the streams.

- The commit message of `Skip t9020 with MSys2`, `t9116: work around
  hard-to-debug hangs` and `t9001: work around hard-to-debug hangs` were
  improved to be a lot more informative, the latter two were also
  combined into a single patch.

- The patch `mingw: support spawning programs containing spaces in their
  names` was augmented with a regression test, and the commit message
  now mentions that this is needed for older Windows versions.

This commit starts the rebase of 4c87c4e to 7a95a1cd084c

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>