URL based configuration for git exec timeouts and git submodule cloning

Adds a number of user-accessable options for configuring internal git resource
cloning behavior.
- Git commands are executed with a configurable timeout by including a parameter
  like "?timeout=2m30s" in the resource URL. This can improve cloning a large
  repository, or over a slow network.
- Git submodule cloning can be disabled by including a parameter like
  "?submodules=false" in the resource URL.
- Switch the overall query parsing to use url.Parse() and be more extensible.

api/internal/git/cloner.go

@@ -14,7 +14,7 @@ type Cloner func(repoSpec *RepoSpec) error
 // to say, some remote API, to obtain a local clone of
 // a remote repo.
 func ClonerUsingGitExec(repoSpec *RepoSpec) error {
-	r, err := newCmdRunner()
+	r, err := newCmdRunner(repoSpec.Timeout)
 	if err != nil {
 		return err
 	}
@@ -36,7 +36,10 @@ func ClonerUsingGitExec(repoSpec *RepoSpec) error {
 	if err = r.run("checkout", "FETCH_HEAD"); err != nil {
 		return err
 	}
-	return r.run("submodule", "update", "--init", "--recursive")
+	if repoSpec.Submodules {
+		return r.run("submodule", "update", "--init", "--recursive")
+	}
+	return nil
 }
 
 // DoNothingCloner returns a cloner that only sets