Add .cipd_impl.ps1 to recipe bundle

https://crrev.com/c/3686855 missed this file to run cipd.bat.

Bug: 931218
Change-Id: Ib4f8d9d31c259477d5531f32bf1e6f44d71d2ace
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3697956
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>

.gitattributes

@@ -31,7 +31,8 @@
 
 # Additional files for cipd_bin_setup.{sh, bat},
 # including cipd/cipd.bat, cipd_client_version* and cipd_manifest*.
-/cipd* recipes
+/cipd*          recipes
+/.cipd_impl.ps1 recipes
 
 # Symlinks
 /cbuildbot      recipes