feature: support git credential helpers

CHANGELOG.md

@@ -62,6 +62,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * The username and hostname that appear in the operation log are now
   configurable via config options `operation.username` and `operation.hostname`.
 
+* `jj git` subcommands now support credential helpers.
+
 ### Fixed bugs
 
 * (#463) A bug in the export of branches to Git caused spurious conflicted

docs/git-compatibility.md

@@ -20,8 +20,8 @@ a comparison with Git, including how workflows are different, see the
   you miss any particular configuration options.
   * The configuration of remotes (`[remote "<name>"]`).
   * `core.excludesFile`
-* **Authentication: Partial.** Only `ssh-agent` or a password-less key file at
-  `~/.ssh/id_rsa` (and only at exactly that path).
+* **Authentication: Partial.** Only `ssh-agent`, a password-less key file at
+  `~/.ssh/id_rsa` (and only at exactly that path), or a `credential.helper`.
 * **Branches: Yes.** You can read more about
   [how branches work in Jujutsu](branches.md)
   and [how they interoperate with Git](#branches).

lib/src/git.rs

@@ -553,7 +553,12 @@ impl<'a> RemoteCallbacks<'a> {
         // TODO: We should expose the callbacks to the caller instead -- the library
         // crate shouldn't read environment variables.
         callbacks.credentials(move |url, username_from_url, allowed_types| {
-            if let Some(username) = username_from_url {
+            let git_config = git2::Config::open_default();
+            let credential_helper = git_config
+                .and_then(|conf| git2::Cred::credential_helper(&conf, url, username_from_url));
+            if let Ok(creds) = credential_helper {
+                return Ok(creds);
+            } else if let Some(username) = username_from_url {
                 if allowed_types.contains(git2::CredentialType::SSH_KEY) {
                     if std::env::var("SSH_AUTH_SOCK").is_ok()
                         || std::env::var("SSH_AGENT_PID").is_ok()