Bug: Changing folder case with `git mv` crashes on case-insensitive file system

[Mark Amery <markrobertamery@gmail.com>]

Attempting to change the case of a folder's name using a command like
`git mv foo FOO` crashes on case-insensitive file systems, like the
default APFS used on Apple Macs.

Here are simple steps to repro this:

    $ mkdir testrepo && cd testrepo && git init
    Initialized empty Git repository in /Users/markamery/testrepo/.git/
    $ mkdir foo && touch foo/bar && git add foo && git commit -m bla
    [master (root-commit) a7e9f5f] bla
    1 file changed, 0 insertions(+), 0 deletions(-)
    create mode 100644 foo/bar
    $ git mv foo FOO
    fatal: renaming 'foo' failed: Invalid argument
    $ echo $?
    128
    $ git status
    On branch master
    nothing to commit, working tree clean

If I create a case-sensitive APFS volume using Disk Utility and try
the commands above on that volume, `git mv foo FOO` works correctly:
it emits no output, exits with a 0 status code, and stages a change
renaming `foo/bar` to `FOO/bar`. However, on my main case-insensitive
volume, `git mv` behaves as shown above: it exits with code 128,
prints an "Invalid argument" error message, and does not stage any
changes.

The command still fails in the same way if you use `git mv --force`
instead of just `git mv`.

Note that previously, `git mv` could not change the case of *file*
names on case-insensitive file systems, until that was fixed in commit
https://github.com/git/git/commit/baa37bff9a845471754d3f47957d58a6ccc30058.
I'm guessing there's a different code path that needs fixing for
changing the case of *folders*.

As far as I can tell, this error has never been reported to the Git
mailing list, but it seems to be encountered frequently;
https://stackoverflow.com/questions/3011625/git-mv-and-only-change-case-of-directory
mentions this bug and has 86000 views.

In case it's relevant, here's my system info as output by `git bugreport`:

    [System Info]
    git version:
    git version 2.31.1
    cpu: x86_64
    no commit associated with this build
    sizeof-long: 8
    sizeof-size_t: 8
    shell-path: /bin/sh
    uname: Darwin 18.7.0 Darwin Kernel Version 18.7.0: Mon Apr 27
20:09:39 PDT 2020; root:xnu-4903.278.35~1/RELEASE_X86_64 x86_64
    compiler info: clang: 11.0.0 (clang-1100.0.33.17)
    libc info: no libc information available
    $SHELL (typically, interactive shell): /bin/bash