Fix checkoutExists and add test for it

Author: ViktorSimko

Sources/SourceControl/GitRepository.swift

@@ -95,7 +95,7 @@ public class GitRepositoryProvider: RepositoryProvider {
     public func checkoutExists(at path: AbsolutePath) throws -> Bool {
         precondition(exists(path))
 
-        let result = try Process.popen(args: Git.tool, "rev-parse", "--is-bare-repository")
+        let result = try Process.popen(args: Git.tool, "-C", path.asString, "rev-parse", "--is-bare-repository")
         return try result.exitStatus == .terminated(code: 0) && result.utf8Output().chomp() == "false"
     }
 

Tests/SourceControlTests/GitRepositoryTests.swift

@@ -40,6 +40,7 @@ class GitRepositoryTests: XCTestCase {
             // Test the provider.
             let testCheckoutPath = path.appending(component: "checkout")
             let provider = GitRepositoryProvider()
+            XCTAssertTrue(try provider.checkoutExists(at: testRepoPath))
             let repoSpec = RepositorySpecifier(url: testRepoPath.asString)
             try! provider.fetch(repository: repoSpec, to: testCheckoutPath)