mingw: skip ownership check on FAT32

The FAT file system has no concept of ACLs. Therefore, it cannot store
any ownership information anyway, and the `GetNamedSecurityInfoW()` call
pretends that everything is owned "by the world".

Let's special-case that scenario and not bother the user with an issue
that cannot be fixed using that file system anyway.

This addresses #3886

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

compat/mingw.c

@@ -3486,6 +3486,22 @@ static PSID get_current_user_sid(void)
 	return result;
 }
 
+static int acls_supported(const char *path)
+{
+	size_t offset = offset_1st_component(path);
+	WCHAR wroot[MAX_PATH];
+	DWORD file_system_flags;
+
+	if (offset &&
+	    xutftowcs_path_ex(wroot, path, MAX_PATH, offset,
+	    		      MAX_PATH, 0) > 0 &&
+	    GetVolumeInformationW(wroot, NULL, 0, NULL, NULL,
+				  &file_system_flags, NULL, 0))
+		return !!(file_system_flags & FILE_PERSISTENT_ACLS);
+
+	return 0;
+}
+
 int is_path_owned_by_current_sid(const char *path)
 {
 	WCHAR wpath[MAX_PATH];
@@ -3541,6 +3557,13 @@ int is_path_owned_by_current_sid(const char *path)
 			 * okay, too.
 			 */
 			result = 1;
+		else if (IsWellKnownSid(sid, WinWorldSid) &&
+			 !acls_supported(path))
+			/*
+			 * On FAT32 volumes, ownership is not actually recorded.
+			 * Simply ignore this situation and continue.
+			 */
+			result = 1;
 		else if (git_env_bool("GIT_TEST_DEBUG_UNSAFE_DIRECTORIES", 0)) {
 			LPSTR str1, str2, to_free1 = NULL, to_free2 = NULL;