[upload] Switch default target branch to "main"

This should resolve the linked bug and be fairly low-risk since the vast
majority of fuchsia-owned repos have main branches.

Fixed: 327615030
Change-Id: I7b91854a927a286707cf569b37ae2a7d76813c90
Reviewed-on: https://fuchsia-review.googlesource.com/c/jiri/+/1035792
Fuchsia-Auto-Submit: Oliver Newman <olivernewman@google.com>
Reviewed-by: Danielle Kay <danikay@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
diff --git a/cmd/jiri/upload.go b/cmd/jiri/upload.go
index 5cfffde..237abbd 100644
--- a/cmd/jiri/upload.go
+++ b/cmd/jiri/upload.go
@@ -201,7 +201,7 @@
 			}
 		}
 		if remoteBranch == "" { // Un-tracked branch
-			remoteBranch = "master"
+			remoteBranch = "main"
 			if r, ok := remoteProjects[project.Key()]; ok {
 				remoteBranch = r.RemoteBranch
 			} else {