git fetch: Refactor a reusable git_fetch function from the command.

We will need this in the `jj git sync` command to perform a fetch that behaves
the same as the `jj git fetch` command.

Fetch extract the function in this commit, so diffs are easier to review.

A later commit will move this function (and associated functions) out to cli/src/git_util.rs

Part of: #1039

cli/src/commands/git/fetch.rs

@@ -69,22 +69,44 @@ pub fn cmd_git_fetch(
         args.remotes.clone()
     };
     let mut tx = workspace_command.start_transaction();
-    for remote in &remotes {
+    git_fetch(
+        ui,
+        &mut tx,
+        command.settings(),
+        &git_repo,
+        &remotes,
+        &args.branch,
+    )?;
+    tx.finish(
+        ui,
+        format!("fetch from git remote(s) {}", remotes.iter().join(",")),
+    )?;
+    Ok(())
+}
+
+fn git_fetch(
+    ui: &mut Ui,
+    tx: &mut WorkspaceCommandTransaction,
+    settings: &UserSettings,
+    git_repo: &git2::Repository,
+    remotes: &[String],
+    branch: &[StringPattern],
+) -> Result<(), CommandError> {
+    for remote in remotes {
         let stats = with_remote_git_callbacks(ui, None, |cb| {
             git::fetch(
                 tx.repo_mut(),
-                &git_repo,
+                git_repo,
                 remote,
-                &args.branch,
+                branch,
                 cb,
-                &command.settings().git_settings(),
+                &settings.git_settings(),
                 None,
             )
         })
         .map_err(|err| match err {
             GitFetchError::InvalidBranchPattern => {
-                if args
-                    .branch
+                if branch
                     .iter()
                     .any(|pattern| pattern.as_exact().map_or(false, |s| s.contains('*')))
                 {
@@ -104,15 +126,10 @@ pub fn cmd_git_fetch(
     }
     warn_if_branches_not_found(
         ui,
-        &tx,
-        &args.branch,
+        tx,
+        branch,
         &remotes.iter().map(StringPattern::exact).collect_vec(),
-    )?;
-    tx.finish(
-        ui,
-        format!("fetch from git remote(s) {}", remotes.iter().join(",")),
-    )?;
-    Ok(())
+    )
 }
 
 const DEFAULT_REMOTE: &str = "origin";