Disable merged commit pre-upload checks by default

When merge-commits are uploaded to Gerrit, the merged commits have
usually either already been reviewed elsewhere (in another branch), or
may contain automatically-generated commits that would probably not pass
the pre-upload checks (e.g. lacking a Bug: tag). This change makes it so
that the merge commits are not checked by the hooks, but the
merge-commit itself is.
Bug: 31926893
Test: repo upload
Change-Id: Ic25127fdbaea651cd4bb14e250c26ffe9aa064ad

GLOBAL-PREUPLOAD.cfg

@@ -2,6 +2,9 @@
 # listed in the manifest.  Do not turn on things here without first consulting
 # the wider Android team, and make sure you send out a PSA before landing.
 
+[Options]
+ignore_merged_commits = true
+
 [Builtin Hooks]
 commit_msg_test_field = true