test-chmtime: work around Windows limitation

Windows has problems changing the mtime when the file is write protected,
even by the owner of said file.

Add a Windows-only workaround to change the mode if necessary before
trying to change the mtime.

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

test-chmtime.c

@@ -87,6 +87,15 @@ int main(int argc, const char *argv[])
 			return -1;
 		}
 
+#ifdef WIN32
+		if (!(sb.st_mode & S_IWUSR) &&
+				chmod(argv[i], sb.st_mode | S_IWUSR)) {
+			fprintf(stderr, "Could not make user-writable %s: %s",
+				argv[i], strerror(errno));
+			return -1;
+		}
+#endif
+
 		utb.actime = sb.st_atime;
 		utb.modtime = set_eq ? set_time : sb.st_mtime + set_time;