newt: Fix shallow upgrade with specific commit version

Upgrading project using shallow option was causing an error if
repository version was specified as specific commit.
Because this specific commit could not yet be fetched
it could not be found during version validation.
Now if specific commit could not be found, instead of returning
an error immediately, we first try to fetch it.

newt/downloader/downloader.go

@@ -61,6 +61,9 @@ type Downloader interface {
 	// Fetches all remotes.
 	Fetch(path string) error
 
+	// Fetches specific commit
+	FetchCommit(path string, commit string) error
+
 	// Checks out the specified commit (hash, tag, or branch).  Always puts the
 	// repo in a "detached head" state.
 	Checkout(path string, commit string) error
@@ -774,6 +777,11 @@ func (gd *GithubDownloader) Fetch(repoDir string) error {
 	})
 }
 
+func (gd *GithubDownloader) FetchCommit(repoDir string, commit string) error {
+	_, err := executeGitCommand(repoDir, []string{"fetch", "--depth=1", "origin", commit}, true)
+	return err
+}
+
 func (gd *GithubDownloader) password() string {
 	if gd.Password != "" {
 		return gd.Password
@@ -943,6 +951,11 @@ func (gd *GitDownloader) Fetch(repoDir string) error {
 	})
 }
 
+func (gd *GitDownloader) FetchCommit(repoDir string, commit string) error {
+	_, err := executeGitCommand(repoDir, []string{"fetch", "--depth=1", "origin", commit}, true)
+	return err
+}
+
 func (gd *GitDownloader) FetchFile(
 	commit string, path string, filename string, dstDir string) error {
 
@@ -1043,6 +1056,11 @@ func (ld *LocalDownloader) Fetch(path string) error {
 	return ld.Clone(ld.MainBranch(), path)
 }
 
+func (ld *LocalDownloader) FetchCommit(path string, commit string) error {
+	_, err := executeGitCommand(path, []string{"fetch", "--depth=1", "origin", commit}, true)
+	return err
+}
+
 func (ld *LocalDownloader) Checkout(path string, commit string) error {
 	_, err := executeGitCommand(path, []string{"checkout", commit}, true)
 	return err

newt/repo/version.go

@@ -126,6 +126,15 @@ func (r *Repo) VersionIsValid(ver newtutil.RepoVersion) bool {
 	}
 
 	cs, _ := r.downloader.CommitsFor(r.Path(), ver.Commit)
+
+	// Try to fetch commit if it was not found
+	if len(cs) <= 0 {
+		err := r.Downloader().FetchCommit(r.Path(), ver.Commit)
+		if err != nil {
+			return false
+		}
+		cs, _ = r.downloader.CommitsFor(r.Path(), ver.Commit)
+	}
 	return len(cs) > 0
 }