Skip fetching external repos if version is SHA

If external repo has version specified as commit SHA and that SHA is
currently checked out, we don't need to fetch that repo.

newt/downloader/downloader.go

@@ -88,6 +88,12 @@ type Downloader interface {
 	// the repo is in a "detached head" state.
 	CurrentBranch(path string) (string, error)
 
+	// Retrieves commit SHA for current HEAD
+	CurrentHead() string
+
+	// Retrieves full SHA for given commit
+	CommitSha(path string, commit string) (string, error)
+
 	// LatestRc finds the commit of the latest release candidate.  It looks
 	// for commits with names matching the base commit string, but with with
 	// "_rc#" inserted.  This is useful when a release candidate is being
@@ -588,6 +594,13 @@ func (gd *GenericDownloader) HashFor(path string,
 		return c.hash, nil
 	}
 
+	if len(commit) < 40 {
+		sha, err := gd.CommitSha(path, commit)
+		if err == nil {
+			commit = sha
+		}
+	}
+
 	return commit, nil
 }
 
@@ -664,6 +677,20 @@ func (gd *GenericDownloader) CurrentBranch(path string) (string, error) {
 	return branch, nil
 }
 
+func (gd *GenericDownloader) CurrentHead() string {
+	return gd.head
+}
+
+func (gd *GenericDownloader) CommitSha(path string, commit string) (string, error) {
+	cmd := []string{"rev-parse", commit}
+	o, err := executeGitCommand(path, cmd, true)
+	if err != nil {
+		return "", err
+	}
+
+	return strings.TrimSpace(string(o)), nil
+}
+
 // Fetches the downloader's origin remote if it hasn't been fetched yet during
 // this run.
 func (gd *GenericDownloader) cachedFetch(fn func() error) error {

newt/project/project.go

@@ -593,6 +593,11 @@ func (proj *Project) downloadRepositoryYmlFiles() error {
 					"External repository \"%s\" does not specify valid commit version (%s)",
 					r.Name(), ver.String())
 			}
+
+			// No need to fetch if requested commit is already checked out
+			if r.IsHeadCommit(ver.Commit) {
+				continue
+			}
 		}
 
 		if _, err := r.UpdateDesc(); err != nil {

newt/repo/version.go

@@ -22,6 +22,7 @@
 package repo
 
 import (
+	"mynewt.apache.org/newt/newt/downloader"
 	"strings"
 
 	log "github.com/sirupsen/logrus"
@@ -119,6 +120,19 @@ func (r *Repo) CommitFromVer(ver newtutil.RepoVersion) (string, error) {
 	return commit, nil
 }
 
+func (r *Repo) IsHeadCommit(commit string) bool {
+	ct, _ := r.downloader.CommitType(r.Path(), commit)
+
+	if ct != downloader.COMMIT_TYPE_HASH {
+		return false
+	}
+
+	head := r.downloader.CurrentHead()
+	sha, _ := r.downloader.CommitSha(r.Path(), commit)
+
+	return head == sha
+}
+
 func (r *Repo) VersionIsValid(ver newtutil.RepoVersion) bool {
 	if ver.Commit == "" {
 		_, err := r.CommitFromVer(ver)