v2.46.0-rc0 test failures on cygwin

The 'Test Summary Report' for the v2.46.0-rc0 test run, on cygwin, includes:

  ...
  t1460-refs-migrate.sh                            (Wstat: 256 (exited 1) Tests: 30 Failed: 9)
    Failed tests:  8-15, 29
    Non-zero exit status: 1
  ...

Looking at the test output directly, we see:
 
  $ pwd
  /home/ramsay/git
  $ cd t
  $ make clean
  rm -f -r 'chainlinttmp'
  rm -f -r 'trash directory'.*
  rm -f -r valgrind/bin
  rm -f -r 'test-results'
  rm -f .prove
  $ ./t1460-refs-migrate.sh -v -i
  ...
  ok 7 - files -> reftable: migration with worktree fails
  
  expecting success of 1460.8 'files -> reftable: unborn HEAD': 
  			test_when_finished "rm -rf repo" &&
  			git init --ref-format=$from_format repo &&
  			test_migration repo "$to_format"
  		
  Initialized empty Git repository in /home/ramsay/git/t/trash directory.t1460-refs-migrate/repo/.git/
  error: could not link file '.git/ref_migration.sr9pEF/reftable' to '.git/reftable': Permission denied
  migrated refs can be found at '.git/ref_migration.sr9pEF'
  not ok 8 - files -> reftable: unborn HEAD
  #	
  #				test_when_finished "rm -rf repo" &&
  #				git init --ref-format=$from_format repo &&
  #				test_migration repo "$to_format"
  #			
  1..8
  $ 

Note that all of the errors in this test look similar to this one (ie. when
rename()-ing the 'migrated' reftable directory to .git/reftable we get an
'Permission denied' error). So the error message is from line 2672 of refs.c
in the move_files() function.

Let's have a quick look at the test directory:

  $ cd trash\ directory.t1460-refs-migrate/
  $ ls
  err  expect  repo/
  $ cd repo
  $ ls -l .git
  total 7.0K
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 branches/
  -rw-r--r-- 1 ramsay None 86 Jul 16 19:53 config
  -rw-r--r-- 1 ramsay None 73 Jul 16 19:53 description
  -rw-r--r-- 1 ramsay None 25 Jul 16 19:53 HEAD
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 hooks/
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 info/
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 objects/
  drwx------ 1 ramsay None  0 Jul 16 19:53 ref_migration.sr9pEF/
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 refs/

Now try to finish the migration by hand:

  $ mv .git/ref_migration.sr9pEF/reftable .git/reftable

Hmm, note no error; of course, the mv command may well do much more than
the rename() library function, so they are not necessarily equivalent.

  $ ls -l .git
  total 7.0K
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 branches/
  -rw-r--r-- 1 ramsay None 86 Jul 16 19:53 config
  -rw-r--r-- 1 ramsay None 73 Jul 16 19:53 description
  -rw-r--r-- 1 ramsay None 25 Jul 16 19:53 HEAD
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 hooks/
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 info/
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 objects/
  drwx------ 1 ramsay None  0 Jul 16 19:55 ref_migration.sr9pEF/
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 refs/
  drwxr-xr-x 1 ramsay None  0 Jul 16 19:53 reftable/
  $ 
  $ rm -rf .git/ref_migration.sr9pEF/
  $ git branch -v
  fatal: failed to resolve HEAD as a valid ref
  $ ls -l .git/reftable
  total 2.0K
  -rw-r--r-- 1 ramsay None 124 Jul 16 19:53 0x000000000001-0x000000000001-64e987ec.ref
  -rw-r--r-- 1 ramsay None  43 Jul 16 19:53 tables.list
  $ xxd .git/reftable/0x000000000001-0x000000000001-64e987ec.ref 
  00000000: 5245 4654 0100 1000 0000 0000 0000 0001  REFT............
  00000010: 0000 0000 0000 0001 7200 0038 0023 4845  ........r..8.#HE
  00000020: 4144 000f 7265 6673 2f68 6561 6473 2f6d  AD..refs/heads/m
  00000030: 6169 6e00 001c 0001 5245 4654 0100 1000  ain.....REFT....
  00000040: 0000 0000 0000 0001 0000 0000 0000 0001  ................
  00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
  00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
  00000070: 0000 0000 0000 0000 b6bf f78a            ............
  $ 

So, the HEAD ref was referring to refs/heads/main.

  $ pwd
  /home/ramsay/git/t/trash directory.t1460-refs-migrate/repo
  $ git init --ref-format=reftable
  fatal: attempt to reinitialize repository with different reference storage format
  $ xxd .git/refs/heads 
  00000000: 7468 6973 2072 6570 6f73 6974 6f72 7920  this repository 
  00000010: 7573 6573 2074 6865 2072 6566 7461 626c  uses the reftabl
  00000020: 6520 666f 726d 6174 0a                   e format.
  $ 
  $ git branch -v
  fatal: failed to resolve HEAD as a valid ref
  $ 
 
Hmm, so quite broken :)

Maybe the order of some of the actions in repo_migrate_ref_storage_format() may
need to be re-thought! ;)

The 'man 3 rename' is not very enlightening; 'The conditions for failure depend
on the host operating system.'

Unfortunately, I don't have any (well much) spare time to dig further into
this at the moment, so I thought I should at least report it here for others
to hopefully find a solution.

ATB,
Ramsay Jones