git: add .jj/.gitignore when creating colocated repo

This replaces our existing mechanism of adding `/.jj/` to
`.git/info/exclude` by adding `*` to `.jj/.gitignore`, as suggested by
@ppwwyyxx. That simplifies the code quite a bit, and it avoids the
problem with `.git/info/exclude` not existing (it apparently doesn't
exist when the user uses
https://git-scm.com/docs/git-init#_template_directory).

Closes #2385.

cli/src/commands/git.rs

@@ -1,5 +1,5 @@
 use std::collections::HashSet;
-use std::io::{Read, Seek as _, SeekFrom, Write};
+use std::io::{Read, Write};
 use std::ops::Deref;
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
@@ -210,43 +210,19 @@ fn map_git_error(err: git2::Error) -> CommandError {
     }
 }
 
-pub fn add_to_git_exclude(ui: &Ui, git_repo: &git2::Repository) -> Result<(), CommandError> {
-    let exclude_file_path = git_repo.path().join("info").join("exclude");
-    if exclude_file_path.exists() {
-        match fs::OpenOptions::new()
-            .read(true)
-            .write(true)
-            .open(&exclude_file_path)
-        {
-            Ok(mut exclude_file) => {
-                let mut buf = vec![];
-                exclude_file.read_to_end(&mut buf)?;
-                let pattern = b"\n/.jj/\n";
-                if !buf.windows(pattern.len()).any(|window| window == pattern) {
-                    exclude_file.seek(SeekFrom::End(0))?;
-                    if !buf.ends_with(b"\n") {
-                        exclude_file.write_all(b"\n")?;
-                    }
-                    exclude_file.write_all(b"/.jj/\n")?;
-                }
-            }
-            Err(err) => {
-                writeln!(
-                    ui.error(),
-                    "Failed to add `.jj/` to {}: {}",
-                    exclude_file_path.to_string_lossy(),
-                    err
-                )?;
-            }
-        }
+pub fn maybe_add_gitignore(workspace_command: &WorkspaceCommandHelper) -> Result<(), CommandError> {
+    if workspace_command.working_copy_shared_with_git() {
+        std::fs::write(
+            workspace_command
+                .workspace_root()
+                .join(".jj")
+                .join(".gitignore"),
+            "/*\n",
+        )
+        .map_err(|e| user_error(format!("Failed to write .jj/.gitignore file: {e}")))
     } else {
-        writeln!(
-            ui.error(),
-            "Failed to add `.jj/` to {} because it doesn't exist",
-            exclude_file_path.to_string_lossy()
-        )?;
+        Ok(())
     }
-    Ok(())
 }
 
 fn cmd_git_remote_add(
@@ -525,7 +501,6 @@ fn do_git_clone(
 ) -> Result<(WorkspaceCommandHelper, GitFetchStats), CommandError> {
     let (workspace, repo) = if colocate {
         let git_repo = git2::Repository::init(wc_path)?;
-        add_to_git_exclude(ui, &git_repo)?;
         Workspace::init_external_git(command.settings(), wc_path, git_repo.path())?
     } else {
         Workspace::init_internal_git(command.settings(), wc_path)?
@@ -537,6 +512,7 @@ fn do_git_clone(
         wc_path.display()
     )?;
     let mut workspace_command = command.for_loaded_repo(ui, workspace, repo)?;
+    maybe_add_gitignore(&workspace_command)?;
     git_repo.remote(remote_name, source).unwrap();
     let mut fetch_tx = workspace_command.start_transaction("fetch from git remote into empty repo");
 

cli/src/commands/mod.rs

@@ -1213,10 +1213,9 @@ fn cmd_init(ui: &mut Ui, command: &CommandHelper, args: &InitArgs) -> Result<(),
             .unwrap()
             .open_git_repo()?;
         let mut workspace_command = command.for_loaded_repo(ui, workspace, repo)?;
+        git::maybe_add_gitignore(&workspace_command)?;
         workspace_command.snapshot(ui)?;
-        if workspace_command.working_copy_shared_with_git() {
-            git::add_to_git_exclude(ui, &git_repo)?;
-        } else {
+        if !workspace_command.working_copy_shared_with_git() {
             let mut tx = workspace_command.start_transaction("import git refs");
             let stats = jj_lib::git::import_some_refs(
                 tx.mut_repo(),

cli/tests/test_git_clone.rs

@@ -224,7 +224,9 @@ fn test_git_clone_colocate() {
         .map(|entry| format!("{:?} {}\n", entry.status(), entry.path().unwrap()))
         .collect();
     insta::assert_snapshot!(git_statuses, @r###"
-    IGNORED .jj/
+    IGNORED .jj/.gitignore
+    IGNORED .jj/repo/
+    IGNORED .jj/working_copy/
     "###);
 
     // The old default branch "master" shouldn't exist.