tests/regres/export_to_sheets: Fix off-by-one in loop

We weren't looking at the most recent change.
Kinda important when that's typically the new test list you're trying to export.

Change-Id: I0e312dbde0ae54feb0641a658eebc73bbdb47eaa
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26928
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>

tests/regres/export_to_sheets.go

@@ -124,7 +124,7 @@ func run() error {
 
 		rowInsertionPoint := 1 + len(hashValues)
 
-		for i := len(changes) - 1; i > 0; i-- {
+		for i := len(changes) - 1; i >= 0; i-- {
 			change := changes[i]
 			if !strings.HasPrefix(change.Subject, consts.TestListUpdateCommitSubjectPrefix) {
 				continue