Fix eng/common cashing (#4829)

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#4751

---------

Co-authored-by: Copilot <[email protected]>

Author: dkurepa

src/Microsoft.DotNet.Darc/DarcLib/GitHubClient.cs

@@ -660,7 +660,10 @@ public async Task<List<GitFile>> GetFilesAtCommitAsync(string repoUri, string co
 
         if (Cache != null)
         {
-            return await Cache.GetOrCreateAsync((treeItem.Path, treeItem.Sha), async (entry) =>
+            // We're adding the full path here because the eng/common files have the same relative path in the VMR
+            // and in product repos relative to the eng/common folder, and we don't want to get bad cache hits.
+            // Their full paths are different so this mitigates the problem
+            return await Cache.GetOrCreateAsync((path, treeItem.Path, treeItem.Sha), async (entry) =>
             {
                 GitFile file = await GetGitItemImpl(path, treeItem, owner, repo);
 

test/Microsoft.DotNet.DarcLib.Tests/GitHubClientTests.cs

@@ -289,7 +289,7 @@ public async Task TreeItemCacheTest(bool enableCache)
             cache.CacheHits.Should().Be(expectedCacheHits);
             // Get it again, this time it should be in the cache
             expectedCacheHits++;
-            await client.Object.GetGitTreeItem("anotherPath", treeItemsToGet[0].Item3, treeItemsToGet[0].Item1, treeItemsToGet[0].Item2);
+            await client.Object.GetGitTreeItem("anotherPath", renamedTreeItem.Item3, renamedTreeItem.Item1, renamedTreeItem.Item2);
             cache.CacheHits.Should().Be(expectedCacheHits);
         }
     }