Keep patch files LF on Windows checkouts

Patches are applied with git apply, which fails on CRLF. With core.autocrlf=true Git also reported every .patch file as modified.

.gitattributes

@@ -0,0 +1,3 @@
+# Patches are applied with `git apply`, which fails on CRLF. Keep them LF even
+# where core.autocrlf=true (Windows checkouts).
+*.patch text eol=lf