mingw: only test index entries for backslashes, not tree entries

During a clone of a repository that contained a file with a backslash in
its name in the past, as of v2.24.1(2), Git for Windows prints errors
like this:

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

The idea is to prevent Git from even trying to write files with
backslashes in their file names: while these characters are valid in
file names on other platforms, on Windows it is interpreted as directory
separator (which would obviously lead to ambiguities, e.g. when there is
a file `a\b` and there is also a file `a/b`).

Arguably, this is the wrong layer for that error: As long as the user
never checks out the files whose names contain backslashes, there should
not be any problem in the first place.

So let's loosen the requirements: we now leave tree entries with
backslashes in their file names alone, but we do require any entries
that are added to the Git index to contain no backslashes on Windows.

Note: just as before, the check is guarded by `core.protectNTFS` (to
allow overriding the check by toggling that config setting), and it
is _only_ performed on Windows, as the backslash is not a directory
separator elsewhere, even when writing to NTFS-formatted volumes.

An alternative approach would be to try to prevent creating files with
backslashes in their file names. However, that comes with its own set of
problems. For example, `git config -f C:\ProgramData\Git\config ...` is
a very valid way to specify a custom config location, and we obviously
do _not_ want to prevent that. Therefore, the approach chosen in this
patch would appear to be better.

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

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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