Add variable to control regres daily branch

This allows a user to run regres daily tests on whatever branch they
want by modifying the gitDailyBranch variable. "HEAD" is the tip of the
master branch, while other values such as "conformance-1.2.6.2" can be
used to test specific branches.

Bug: b/217892550
Change-Id: I6c47f3493bfe15405ae98fa6dc62711b5775abad
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/64249
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Sean Risser <srisser@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Commit-Queue: Sean Risser <srisser@google.com>
diff --git a/tests/regres/cmd/regres/main.go b/tests/regres/cmd/regres/main.go
index 2c75694..8ecddfb 100644
--- a/tests/regres/cmd/regres/main.go
+++ b/tests/regres/cmd/regres/main.go
@@ -61,6 +61,7 @@
 
 const (
 	gitURL                = "https://swiftshader.googlesource.com/SwiftShader"
+	gitDailyBranch        = "HEAD"
 	gerritURL             = "https://swiftshader-review.googlesource.com/"
 	coverageURL           = "https://$USERNAME:$PASSWORD@github.com/swiftshader-regres/swiftshader-coverage.git"
 	coverageBranch        = "gh-pages"
@@ -638,7 +639,7 @@
 
 	dailyHash := git.Hash{}
 	if r.dailyChange == "" {
-		headHash, err := git.FetchRefHash("HEAD", gitURL)
+		headHash, err := git.FetchRefHash(gitDailyBranch, gitURL)
 		if err != nil {
 			return cause.Wrap(err, "Could not get hash of master HEAD")
 		}