pkg/bisect: add initial testing support for cause bisection

(note: incomplete change)

Refactor existing code as follows:
* Move reusable test utility functions from git_repo_test.go to
pkg/vcs/test_util.go and make them exported.
* Split Run() into Run()+runImpl().
* Change type of bisect.go:env.inst to `instance.BuilderTester`.
Change usage inside syz-testbuild/testbuild.go accordingly.
* Move most of linux.PreviousReleaseTags() into vcs/git.go as
git.previousReleaseTags().
* Allow build.CompilerIdentity to be mocked.

Introduce the following changes:
* instance.BuilderTester is an interface with methods
	BuildSyzkaller()
	BuildKernel()
	Test()
NewEnv() now returns this interface.
* type testEnv implements instance.BuilderTester.
* type testBuilder implements builder interface. Add a entry into table
inside pkg/build/build.go:getBuilder() to return testBuilder object.

1 files changed, 1 insertions, 21 deletions

diff --git a/pkg/vcs/linux.go b/pkg/vcs/linux.go
index e12682277..aaca97fe7 100644
--- a/pkg/vcs/linux.go
+++ b/pkg/vcs/linux.go
@@ -31,30 +31,10 @@ func newLinux(dir string) *linux {
 }
 
 func (ctx *linux) PreviousReleaseTags(commit string) ([]string, error) {
-	return ctx.previousReleaseTags(commit, false)
-}
-
-func (ctx *linux) previousReleaseTags(commit string, self bool) ([]string, error) {
-	var tags []string
-	if self {
-		output, err := ctx.git.git("tag", "--list", "--points-at", commit, "--merged", commit, "v*.*")
-		if err != nil {
-			return nil, err
-		}
-		tags, err = gitParseReleaseTags(output)
-		if err != nil {
-			return nil, err
-		}
-	}
-	output, err := ctx.git.git("tag", "--no-contains", commit, "--merged", commit, "v*.*")
-	if err != nil {
-		return nil, err
-	}
-	tags1, err := gitParseReleaseTags(output)
+	tags, err := ctx.git.previousReleaseTags(commit, false)
 	if err != nil {
 		return nil, err
 	}
-	tags = append(tags, tags1...)
 	for i, tag := range tags {
 		if tag == "v4.0" {
 			// Initially we tried to stop at 3.8 because: