[roll third_party/pigweed] pw_presubmit: Default --base to tracking branch

Default 'pw presubmit' to using the current branch's tracking branch as
the base commit for finding changed files (instead of running on all
files).

Original-Bug: 588
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/76102

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 66c509979444b2d..b839d428445ebad
Roller-URL: https://ci.chromium.org/b/8825838142003282369
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8825838142003282369
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I9dce2eed7937dcf75929888c0dd314ff69860c5c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/76940
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 66c5099..b839d42 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 66c509979444b2de1954a12b9a8988066f5c65af
+Subproject commit b839d428445ebad1119f6337cee56c3a5f1574da