cmd/go: explicitly name the default branch “main” in testdata/vcstest/git/issue57650.txt

Updates #47650.

Change-Id: I90c0a51b390326129643d5b90fe935d44e5e5372
Reviewed-on: https://go-review.googlesource.com/c/go/+/549155
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: Bryan C. Mills

Diff for: src/cmd/go/testdata/vcstest/git/issue47650.txt

@@ -10,6 +10,7 @@ git init
 at 2021-08-11T13:52:00-04:00
 git add cmd
 git commit -m 'add cmd/issue47650'
+git branch -m main
 git tag v0.1.0
 
 git add go.mod