… backslashes

As of Git for Windows, v2.24.1(2). cloning a repository that contained a
file with a backslash in its name some time in the past, the command will
succeed but at the same time print errors like this:

    error: filename in tree entry contains backslash: '\'

A corresponding git fetch will also show those errors, but fail.

The reason is that v2.24.1 is much more strict about backslashes in tree
entries than earlier versions. The intention was actually to prevent
checking out such files, though: if there was a mistake in a repository long
ago that has been fixed long since, there is actually no reason why we
should require the history to be rewritten.

This fixes https://github.com/git-for-windows/git/issues/2435.

The idea of this patch is to stop complaining about tree entries, and focus
instead on the index: whenever a file is added to the index, we do not want
any backslashes in the file name on Windows.

As before, this check is only performed on Windows, and only under
core.protectNTFS. On other platforms, even if core.protectNTFS is turned on,
the backslash is not a directory separator, therefore the Operating System's
syscalls will (should?) refuse to create files on NTFS with backslashes in
their file name.

I would appreciate reviews with a particular eye on keeping users safe: I am
not 100% certain that all relevant file writes go through the index (I think
that they all go through the index, but I might have well missed a corner
case).

Johannes Schindelin (1):
  mingw: only test index entries for backslashes, not tree entries

 read-cache.c               | 5 +++++
 t/t7415-submodule-names.sh | 7 ++++---
 tree-walk.c                | 6 ------
 3 files changed, 9 insertions(+), 9 deletions(-)

base-commit: 12029dc57db23baef008e77db1909367599210ee

Submitted-As: https://lore.kernel.org/git/pull.682.git.git.1577382151.gitgitgadget@gmail.com