only update local checkout if necessary (#6965)

motivation: reduce redundant git operations

changes:
* before updating a local checkout, confirm the action is nevessary
* update test to check for revision and not tag

Author: tomerd

Sources/Workspace/Workspace.swift

@@ -3751,10 +3751,14 @@ extension Workspace {
                     break fetch
                 }
 
-                // we need do mutable-immutable dance.
-                try self.fileSystem.chmod(.userWritable, path: checkoutPath, options: [.recursive, .onlyFiles])
-                try workingCopy.fetch()
-                try? self.fileSystem.chmod(.userUnWritable, path: checkoutPath, options: [.recursive, .onlyFiles])
+                // only update if necessary
+                if !workingCopy.exists(revision: revision) {
+                    // The fetch operation may update contents of the checkout, 
+                    // so we need to do mutable-immutable dance.
+                    try self.fileSystem.chmod(.userWritable, path: checkoutPath, options: [.recursive, .onlyFiles])
+                    try workingCopy.fetch()
+                    try? self.fileSystem.chmod(.userUnWritable, path: checkoutPath, options: [.recursive, .onlyFiles])
+                }
 
                 return checkoutPath
             }

Tests/CommandsTests/PackageToolTests.swift

@@ -264,17 +264,35 @@ final class PackageToolTests: CommandsTestCase {
 
             // Perform an initial fetch.
             _ = try execute(["resolve"], packagePath: packageRoot)
-            var path = try SwiftPM.packagePath(for: "Foo", packageRoot: packageRoot)
-            XCTAssertEqual(try GitRepository(path: path).getTags(), ["1.2.3"])
 
-            // Retag the dependency, and update.
-            let repo = GitRepository(path: fixturePath.appending("Foo"))
+            do {
+                let checkoutPath = try SwiftPM.packagePath(for: "Foo", packageRoot: packageRoot)
+                let checkoutRepo = GitRepository(path: checkoutPath)
+                XCTAssertEqual(try checkoutRepo.getTags(), ["1.2.3"])
+                _ = try checkoutRepo.revision(forTag: "1.2.3")
+            }
+
+
+            // update and retag the dependency, and update.
+            let repoPath = fixturePath.appending("Foo")
+            let repo = GitRepository(path: repoPath)
+            try localFileSystem.writeFileContents(repoPath.appending("test"), string: "test")
+            try repo.stageEverything()
+            try repo.commit()
             try repo.tag(name: "1.2.4")
+
+            // we will validate it is there
+            let revision = try repo.revision(forTag: "1.2.4")
+
             _ = try execute(["update"], packagePath: packageRoot)
 
-            // We shouldn't assume package path will be same after an update so ask again for it.
-            path = try SwiftPM.packagePath(for: "Foo", packageRoot: packageRoot)
-            XCTAssertEqual(try GitRepository(path: path).getTags(), ["1.2.3", "1.2.4"])
+            do {
+                // We shouldn't assume package path will be same after an update so ask again for it.
+                let checkoutPath = try SwiftPM.packagePath(for: "Foo", packageRoot: packageRoot)
+                let checkoutRepo = GitRepository(path: checkoutPath)
+                // tag may not be there, but revision should be after update
+                XCTAssertTrue(checkoutRepo.exists(revision: .init(identifier: revision)))
+            }
         }
     }