Fix infinite searching for .swift-format file on Linux

Due to swiftlang/swift-foundation#980, the we may end up with a `path == ""` instead of `/`.

We didn’t catch this in the test because we only end up with an empty path when searching for a `.swift-format` file from a `.swift-file` that was not at the root of a directory.

Sources/SwiftFormat/API/Configuration.swift

@@ -480,7 +480,9 @@ fileprivate extension URL {
     // https://github.com/swiftlang/swift-format/issues/844
     return self.pathComponents.count == 1
     #else
-    return self.path == "/"
+    // On Linux, we may end up with an string for the path due to https://github.com/swiftlang/swift-foundation/issues/980
+    // TODO: Remove the check for "" once https://github.com/swiftlang/swift-foundation/issues/980 is fixed.
+    return self.path == "/" || self.path == ""
     #endif
   }
 }

Tests/SwiftFormatTests/API/ConfigurationTests.swift

@@ -26,4 +26,13 @@ final class ConfigurationTests: XCTestCase {
     #endif
     XCTAssertNil(Configuration.url(forConfigurationFileApplyingTo: URL(fileURLWithPath: path)))
   }
+
+  func testMissingConfigurationFileInSubdirectory() {
+    #if os(Windows)
+    let path = #"C:\whatever\test.swift"#
+    #else
+    let path = "/whatever/test.swift"
+    #endif
+    XCTAssertNil(Configuration.url(forConfigurationFileApplyingTo: URL(fileURLWithPath: path)))
+  }
 }