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: '\'

While the clone still succeeds, a similar error prevents the equivalent
`git fetch` operation, which is inconsistent.

Arguably, this is the wrong layer for that error, anyway: 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 instead prevent such files to be added to the index.

This addresses git-for-windows#2435

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

read-cache.c

@@ -1278,6 +1278,11 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e
 	int skip_df_check = option & ADD_CACHE_SKIP_DFCHECK;
 	int new_only = option & ADD_CACHE_NEW_ONLY;
 
+#ifdef GIT_WINDOWS_NATIVE
+	if (protect_ntfs && strchr(ce->name, '\\'))
+		return error(_("filename in tree entry contains backslash: '%s'"), ce->name);
+#endif
+
 	if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
 		cache_tree_invalidate_path(istate, ce->name);
 

t/t7415-submodule-names.sh

@@ -207,16 +207,17 @@ test_expect_success MINGW 'prevent git~1 squatting on Windows' '
 			git hash-object -w --stdin)" &&
 		rev="$(git rev-parse --verify HEAD)" &&
 		hash="$(echo x | git hash-object -w --stdin)" &&
+		test_must_fail git update-index --add \
+			--cacheinfo 160000,$rev,d\\a 2>err &&
+		test_i18ngrep backslash err &&
 		git -c core.protectNTFS=false update-index --add \
 			--cacheinfo 100644,$modules,.gitmodules \
 			--cacheinfo 160000,$rev,c \
 			--cacheinfo 160000,$rev,d\\a \
 			--cacheinfo 100644,$hash,d./a/x \
 			--cacheinfo 100644,$hash,d./a/..git &&
 		test_tick &&
-		git -c core.protectNTFS=false commit -m "module" &&
-		test_must_fail git show HEAD: 2>err &&
-		test_i18ngrep backslash err
+		git -c core.protectNTFS=false commit -m "module"
 	) &&
 	test_must_fail git -c core.protectNTFS=false \
 		clone --recurse-submodules squatting squatting-clone 2>err &&

tree-walk.c

@@ -43,12 +43,6 @@ static int decode_tree_entry(struct tree_desc *desc, const char *buf, unsigned l
 		strbuf_addstr(err, _("empty filename in tree entry"));
 		return -1;
 	}
-#ifdef GIT_WINDOWS_NATIVE
-	if (protect_ntfs && strchr(path, '\\')) {
-		strbuf_addf(err, _("filename in tree entry contains backslash: '%s'"), path);
-		return -1;
-	}
-#endif
 	len = strlen(path) + 1;
 
 	/* Initialize the descriptor entry */