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, 24 insertions, 0 deletions

diff --git a/pkg/vcs/git.go b/pkg/vcs/git.go
index 7b0a621b6..359c3d68e 100644
--- a/pkg/vcs/git.go
+++ b/pkg/vcs/git.go
@@ -450,3 +450,27 @@ func (git *git) bisectInconclusive(output []byte) ([]*Commit, error) {
 	}
 	return commits, nil
 }
+
+func (git *git) previousReleaseTags(commit string, self bool) ([]string, error) {
+	var tags []string
+	if self {
+		output, err := 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 := git.git("tag", "--no-contains", commit, "--merged", commit, "v*.*")
+	if err != nil {
+		return nil, err
+	}
+	tags1, err := gitParseReleaseTags(output)
+	if err != nil {
+		return nil, err
+	}
+	tags = append(tags, tags1...)
+	return tags, nil
+}