Test formatDiff in a parallel build of several projects

formatDiff fails there as the Spotless step did: with the Java-based
formatter, a subproject's formatDiff resolves the root project's
palantirJavaFormat as its classpath, without the root project's lock,
and Gradle refuses. With the configuration cache the build fails as
soon as the entry is stored.

Author: abashev

gradle-open-java-format/src/test/java/com/palantir/javaformat/gradle/MultiProjectTest.java

@@ -147,6 +147,24 @@ void nothing_is_resolved_for_a_build_that_does_not_format() {
         project.succeeds("help");
     }
 
+    @Test
+    void formatDiff_formats_every_subproject_in_a_parallel_build() throws IOException, InterruptedException {
+        var project =
+                project().writeFile("a/" + MAIN_JAVA, "class Main {}\n").writeFile("b/" + MAIN_JAVA, "class Main {}\n");
+        git(project, "init");
+        git(project, "config", "user.name", "Foo");
+        git(project, "config", "user.email", "foo@bar.com");
+        // The repository this runs in may sign commits; a throwaway one has no key to sign with.
+        git(project, "config", "commit.gpgsign", "false");
+        git(project, "add", ".");
+        git(project, "commit", "-m", "Commit");
+        project.writeFile("a/" + MAIN_JAVA, UNFORMATTED).writeFile("b/" + MAIN_JAVA, UNFORMATTED);
+
+        project.succeeds("formatDiff", "--parallel");
+
+        assertFormatted(project, "a/" + MAIN_JAVA, "b/" + MAIN_JAVA);
+    }
+
     /** The layout of the docs, with no java on the root project. */
     private GradleTestProject project() {
         return new GradleTestProject(projectDir)
@@ -230,4 +248,16 @@ private static void write(Path file, String content) throws IOException {
             Files.writeString(file, content);
         }
     }
+
+    private static void git(GradleTestProject project, String... args) throws IOException, InterruptedException {
+        var command = new String[args.length + 1];
+        command[0] = "git";
+        System.arraycopy(args, 0, command, 1, args.length);
+        var process =
+                new ProcessBuilder(command).directory(project.path().toFile()).start();
+        var exitCode = process.waitFor();
+        if (exitCode != 0) {
+            throw new RuntimeException("git " + String.join(" ", args) + " failed with exit code " + exitCode);
+        }
+    }
 }