cli: respect .git/info/exclude if in Git-backed repo (#65)

Closes #65.

src/commands.rs

@@ -390,8 +390,10 @@ impl WorkspaceCommandHelper {
             // hard-coding its name like this.
             git_ignores = git_ignores.chain_with_file("", home_dir_path.join(".gitignore"));
         }
-        // TODO: Chain with the .jj/git/info/exclude file if we're inside a git-backed
-        // repo.
+        if let Some(git_repo) = self.repo.store().git_repo() {
+            git_ignores =
+                git_ignores.chain_with_file("", git_repo.path().join("info").join("exclude"));
+        }
         git_ignores
     }
 

tests/test_gitignores.rs

@@ -0,0 +1,54 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::io::Write;
+
+use jujutsu::testutils::{get_stdout_string, TestEnvironment};
+
+#[test]
+fn test_gitignores() {
+    let test_env = TestEnvironment::default();
+    let workspace_root = test_env.env_root().join("repo");
+    git2::Repository::init(&workspace_root).unwrap();
+    test_env
+        .jj_cmd(&workspace_root, &["init", "--git-repo", "."])
+        .assert()
+        .success();
+
+    // Say in .git/info/exclude that we don't want file1 and file2
+    let mut file = std::fs::OpenOptions::new()
+        .append(true)
+        .open(workspace_root.join(".git").join("info").join("exclude"))
+        .unwrap();
+    file.write_all(b"file1\nfile2").unwrap();
+    drop(file);
+
+    // Say in .gitignore (in the working copy) that we actually do want file2
+    std::fs::write(workspace_root.join(".gitignore"), "!file2").unwrap();
+
+    // Writes some files to the working copy
+    std::fs::write(workspace_root.join("file0"), "contents").unwrap();
+    std::fs::write(workspace_root.join("file1"), "contents").unwrap();
+    std::fs::write(workspace_root.join("file2"), "contents").unwrap();
+
+    let assert = test_env
+        .jj_cmd(&workspace_root, &["diff", "-s"])
+        .assert()
+        .success();
+    insta::assert_snapshot!(get_stdout_string(&assert), @r###"
+    A .gitignore
+    A file0
+    A file2
+    "###);
+}