git: add git.auto-local-branch

Add a new git.auto-local-branch config option. When set to false, a
remote-tracking branch imported from Git will not automatically create a
local branch target. This is implemented by a new GitSettings struct
that passes Git-related settings from UserSettings.

This behavior is particularly useful in a co-located jj and Git repo,
because a Git remote might have branches that are not of everyday
interest to the user, so it does not make sense to export them as local
branches in Git. E.g. https://github.com/gitster/git, the maintainer's
fork of Git, has 379 branches, most of which are topic branches kept
around for historical reasons, and Git developers wouldn't be expected
to have local branches for each remote-tracking branch.

docs/config.md

@@ -268,6 +268,29 @@ the conflict is done, `jj` assumes that the conflict was only partially resolved
 and parses the conflict markers to get the new state of the conflict. The
 conflict is considered fully resolved when there are no conflict markers left.
 
+## Git settings
+
+### Automatic local branch creation
+
+By default, when `jj` imports a remote-tracking branch from Git, it also
+creates a local branch with the same name. In some repositories, this
+may be undesirable, e.g.:
+
+- There is a remote with a lot of historical branches that you don't
+  want to be exported to the co-located Git repo.
+- There are multiple remotes with conflicting views of that branch,
+  resulting in an unhelpful conflicted state.
+
+You can disable this behavior by setting `git.auto-local-branch` like
+so,
+
+    git.auto-local-branch = false
+
+Note that this setting may make it easier to accidentally delete remote
+branches. Since the local branch isn't created, the remote branch will be
+deleted if you push the branch with `jj git push --branch` or `jj git push
+--all`.
+
 # Alternative ways to specify configuration settings
 
 Instead of `~/.jjconfig.toml`, the config settings can be located at

lib/src/git.rs

@@ -25,6 +25,7 @@ use crate::commit::Commit;
 use crate::git_backend::NO_GC_REF_NAMESPACE;
 use crate::op_store::RefTarget;
 use crate::repo::MutableRepo;
+use crate::settings::GitSettings;
 use crate::view::RefName;
 
 #[derive(Error, Debug, PartialEq)]
@@ -65,6 +66,7 @@ fn prevent_gc(git_repo: &git2::Repository, id: &CommitId) {
 pub fn import_refs(
     mut_repo: &mut MutableRepo,
     git_repo: &git2::Repository,
+    git_settings: &GitSettings,
 ) -> Result<(), GitImportError> {
     let store = mut_repo.store().clone();
     let mut existing_git_refs = mut_repo.view().git_refs().clone();
@@ -141,6 +143,9 @@ pub fn import_refs(
             mut_repo.merge_single_ref(&ref_name, old_git_target.as_ref(), new_git_target.as_ref());
             // If a git remote-tracking branch changed, apply the change to the local branch
             // as well
+            if !git_settings.auto_local_branch {
+                continue;
+            }
             if let RefName::RemoteBranch { branch, remote: _ } = ref_name {
                 mut_repo.merge_single_ref(
                     &RefName::LocalBranch(branch),
@@ -336,6 +341,7 @@ pub fn fetch(
     git_repo: &git2::Repository,
     remote_name: &str,
     callbacks: RemoteCallbacks<'_>,
+    git_settings: &GitSettings,
 ) -> Result<Option<String>, GitFetchError> {
     let mut remote =
         git_repo
@@ -378,7 +384,7 @@ pub fn fetch(
     tracing::debug!("remote.disconnect");
     remote.disconnect()?;
     tracing::debug!("import_refs");
-    import_refs(mut_repo, git_repo).map_err(|err| match err {
+    import_refs(mut_repo, git_repo, git_settings).map_err(|err| match err {
         GitImportError::InternalGitError(source) => GitFetchError::InternalGitError(source),
     })?;
     Ok(default_branch)

lib/src/settings.rs

@@ -33,6 +33,28 @@ pub struct RepoSettings {
     _config: config::Config,
 }
 
+#[derive(Debug, Clone)]
+pub struct GitSettings {
+    pub auto_local_branch: bool,
+}
+
+impl GitSettings {
+    pub fn from_config(config: &config::Config) -> Self {
+        let mut settings = GitSettings::default();
+
+        settings.auto_local_branch = config.get_bool("git.auto-local-branch").unwrap_or(true);
+        settings
+    }
+}
+
+impl Default for GitSettings {
+    fn default() -> Self {
+        GitSettings {
+            auto_local_branch: true,
+        }
+    }
+}
+
 fn get_timestamp_config(config: &config::Config, key: &str) -> Option<Timestamp> {
     match config.get_string(key) {
         Ok(timestamp_str) => match DateTime::parse_from_rfc3339(&timestamp_str) {
@@ -153,6 +175,10 @@ impl UserSettings {
         &self.config
     }
 
+    pub fn git_settings(&self) -> GitSettings {
+        GitSettings::from_config(&self.config)
+    }
+
     pub fn graph_style(&self) -> String {
         self.config
             .get_string("ui.graph.style")