clone: respect remote default branch name

Attempt to determine the remote's default branch name when performing a
`scalar clone` using the vanilla Git protocols (not using the GVFS helper).

Use ls-remote to lookup the HEAD ref on the remote, and parse the
results. If the remote's HEAD is not a branch, or we fail to parse the
output for any reason, consult Git itself for the default branch name as
a fallback.

Author: mjcheetham

Scalar.Common/Git/GitProcess.cs

@@ -604,6 +604,63 @@ public Result RemoteAdd(string remoteName, string url)
             return this.InvokeGitAgainstDotGitFolder("remote add " + remoteName + " " + url);
         }
 
+        public bool TryGetRemoteDefaultBranch(string remoteName, out string defaultBranch, out string error)
+        {
+            defaultBranch = null;
+            error = null;
+
+            Result lsRemoteResult = this.InvokeGitAgainstDotGitFolder("ls-remote --symref origin HEAD");
+            if (lsRemoteResult.ExitCodeIsFailure)
+            {
+                error = "Failed to call ls-remote to determine remote HEAD";
+                return false;
+            }
+
+            string output = lsRemoteResult.Output;
+
+            const string refPrefix = "ref: ";
+            const string headsPrefix = "ref: refs/heads/";
+            const string suffix = "\tHEAD";
+
+            var cmp = StringComparison.Ordinal;
+            foreach (string line in output.Split('\n'))
+            {
+                int suffixStart = line.IndexOf(suffix, cmp);
+                if (line.StartsWith(refPrefix, cmp) && suffixStart > -1)
+                {
+                    // HEAD is a branch
+                    if (line.StartsWith(headsPrefix, cmp))
+                    {
+                        defaultBranch = line.Substring(headsPrefix.Length, suffixStart - headsPrefix.Length);
+                        return true;
+                    }
+
+                    error = "HEAD is not a branch";
+                    return false;
+                }
+            }
+
+            defaultBranch = null;
+            error = $"HEAD not found in ls-remote output: {output}";
+            return false;
+        }
+
+        public bool TryGetSymbolicRef(string name, bool shortName, out string branch, out string error)
+        {
+            branch = null;
+            error = null;
+
+            Result result = this.InvokeGitAgainstDotGitFolder($"symbolic-ref {(shortName ? "--short" : string.Empty)} HEAD");
+            if (result.ExitCodeIsSuccess)
+            {
+                branch = result.Output.Trim();
+                return true;
+            }
+
+            error = $"Failed to read symbolic ref '{name}': {result.Errors}";
+            return false;
+        }
+
         public Result PrunePacked(string gitObjectDirectory)
         {
             return this.InvokeGitAgainstDotGitFolder(

Scalar/CommandLine/CloneVerb.cs

@@ -339,10 +339,6 @@ private Result GitClone()
             git.SetInLocalConfig("remote.origin.promisor", "true");
             git.SetInLocalConfig("remote.origin.partialCloneFilter", "blob:none");
 
-            string branch = this.Branch ?? "master";
-            git.SetInLocalConfig($"branch.{branch}.remote", "origin");
-            git.SetInLocalConfig($"branch.{branch}.merge", $"refs/heads/{branch}");
-
             if (!this.FullClone)
             {
                 GitProcess.SparseCheckoutInit(this.enlistment);
@@ -393,8 +389,25 @@ private Result GitClone()
                 return new Result($"Failed to complete regular clone: {fetchResult?.Errors}");
             }
 
-            GitProcess.Result checkoutResult = null;
+            // Configure the specified branch, or the default branch on the remote if not specified
+            string branch = this.Branch;
+            if (branch is null && !git.TryGetRemoteDefaultBranch("origin", out branch, out string defaultBranchError))
+            {
+                // Failed to get the remote's default branch name - ask Git for the prefered local default branch
+                // instead, and show a warning message.
+                this.Output.WriteLine($"warning: failed to get default branch name from remote; using local default: {defaultBranchError}");
 
+                if (!git.TryGetSymbolicRef("HEAD", shortName: true, out branch, out string localDefaultError))
+                {
+                    return new Result($"Failed to determine local default branch name: {localDefaultError}");
+                }
+            }
+
+            git.SetInLocalConfig($"branch.{branch}.remote", "origin");
+            git.SetInLocalConfig($"branch.{branch}.merge", $"refs/heads/{branch}");
+
+            // Checkout the branch
+            GitProcess.Result checkoutResult = null;
             if (!this.ShowStatusWhileRunning(() =>
                     {
                         using (ITracer activity = this.tracer.StartActivity("git-checkout", EventLevel.LogAlways))