[Checkout] Configure sparse checkouts before fetch (#5106)

This addresses two issues - first, it allows us to avoid fetching files
when checking out a clean repo, as if we pre-configure sparse checkouts
we won't fetch unneeded trees and blobs. Second, it allows us to avoid
not fetching files incorrectly if we're moving a checkout from sparse to
non-sparse.

Fixes #5091

Co-authored-by: Derek Morris <[email protected]>

Author: tarunramsinghani

src/Agent.Plugins/GitSourceProvider.cs

@@ -699,6 +699,31 @@ public async Task GetSourceAsync(
                 }
             }
 
+            if (AgentKnobs.UseSparseCheckoutInCheckoutTask.GetValue(executionContext).AsBoolean())
+            {
+                // Sparse checkout needs to be before any `fetch` task to avoid fetching the excluded trees and blobs, or to not _not_ fetch them if we're disabling a previous sparse checkout.
+                if (enableSparseCheckout)
+                {
+                    // Set up sparse checkout
+                    int exitCode_sparseCheckout = await gitCommandManager.GitSparseCheckout(executionContext, targetPath, sparseCheckoutDirectories, sparseCheckoutPatterns, cancellationToken);
+
+                    if (exitCode_sparseCheckout != 0)
+                    {
+                        throw new InvalidOperationException($"Git sparse checkout failed with exit code: {exitCode_sparseCheckout}");
+                    }
+                }
+                else
+                {
+                    // Disable sparse checkout in case it was enabled in a previous checkout
+                    int exitCode_sparseCheckoutDisable = await gitCommandManager.GitSparseCheckoutDisable(executionContext, targetPath, cancellationToken);
+
+                    if (exitCode_sparseCheckoutDisable != 0)
+                    {
+                        throw new InvalidOperationException($"Git sparse checkout disable failed with exit code: {exitCode_sparseCheckoutDisable}");
+                    }
+                }
+            }
+
             await RunGitStatusIfSystemDebug(executionContext, gitCommandManager, targetPath);
 
             cancellationToken.ThrowIfCancellationRequested();
@@ -993,30 +1018,6 @@ public async Task GetSourceAsync(
                 }
             }
 
-            if (AgentKnobs.UseSparseCheckoutInCheckoutTask.GetValue(executionContext).AsBoolean())
-            {
-                if (enableSparseCheckout)
-                {
-                    // Set up sparse checkout
-                    int exitCode_sparseCheckout = await gitCommandManager.GitSparseCheckout(executionContext, targetPath, sparseCheckoutDirectories, sparseCheckoutPatterns, cancellationToken);
-
-                    if (exitCode_sparseCheckout != 0)
-                    {
-                        throw new InvalidOperationException($"Git sparse checkout failed with exit code: {exitCode_sparseCheckout}");
-                    }
-                }
-                else
-                {
-                    // Disable sparse checkout in case it was enabled in a previous checkout
-                    int exitCode_sparseCheckoutDisable = await gitCommandManager.GitSparseCheckoutDisable(executionContext, targetPath, cancellationToken);
-
-                    if (exitCode_sparseCheckoutDisable != 0)
-                    {
-                        throw new InvalidOperationException($"Git sparse checkout disable failed with exit code: {exitCode_sparseCheckoutDisable}");
-                    }
-                }
-            }
-
             // Finally, checkout the sourcesToBuild (if we didn't find a valid git object this will throw)
             int exitCode_checkout = await gitCommandManager.GitCheckout(executionContext, targetPath, sourcesToBuild, string.Join(" ", additionalCheckoutArgs), cancellationToken);
             if (exitCode_checkout != 0)