git: absence of globs is None rather than &[]

In `git_fetch()`, any glob present in `globs` is an "allow" mark. Using
`&[]` to represent an "allow-all" may be misleading, as it could
indicate that no branch (only the git HEAD) should be fetched.

By using an `Option<&[&str]>`, it is clearer that `None` means that
all branches are fetched.

lib/src/git.rs

@@ -342,7 +342,7 @@ pub fn fetch(
     mut_repo: &mut MutableRepo,
     git_repo: &git2::Repository,
     remote_name: &str,
-    globs: &[&str],
+    branch_name_globs: Option<&[&str]>,
     callbacks: RemoteCallbacks<'_>,
     git_settings: &GitSettings,
 ) -> Result<Option<String>, GitFetchError> {
@@ -364,15 +364,19 @@ pub fn fetch(
     fetch_options.proxy_options(proxy_options);
     let callbacks = callbacks.into_git();
     fetch_options.remote_callbacks(callbacks);
+    let refspecs = if let Some(globs) = branch_name_globs {
         if globs.iter().any(|g| g.contains(|c| ":^".contains(c))) {
             return Err(GitFetchError::InvalidGlob);
         }
         // At this point, we are only updating Git's remote tracking branches, not the
         // local branches.
-    let refspecs = globs
+        globs
             .iter()
             .map(|glob| format!("+refs/heads/{glob}:refs/remotes/{remote_name}/{glob}"))
-        .collect_vec();
+            .collect_vec()
+    } else {
+        vec![]
+    };
     tracing::debug!("remote.download");
     remote.download(&refspecs, Some(&mut fetch_options))?;
     tracing::debug!("remote.prune");

lib/tests/test_git.rs

@@ -957,7 +957,7 @@ fn test_fetch_empty_repo() {
         tx.mut_repo(),
         &test_data.git_repo,
         "origin",
-        &[],
+        None,
         git::RemoteCallbacks::default(),
         &git_settings,
     )
@@ -981,7 +981,7 @@ fn test_fetch_initial_commit() {
         tx.mut_repo(),
         &test_data.git_repo,
         "origin",
-        &[],
+        None,
         git::RemoteCallbacks::default(),
         &git_settings,
     )
@@ -1023,7 +1023,7 @@ fn test_fetch_success() {
         tx.mut_repo(),
         &test_data.git_repo,
         "origin",
-        &[],
+        None,
         git::RemoteCallbacks::default(),
         &git_settings,
     )
@@ -1044,7 +1044,7 @@ fn test_fetch_success() {
         tx.mut_repo(),
         &test_data.git_repo,
         "origin",
-        &[],
+        None,
         git::RemoteCallbacks::default(),
         &git_settings,
     )
@@ -1086,7 +1086,7 @@ fn test_fetch_prune_deleted_ref() {
         tx.mut_repo(),
         &test_data.git_repo,
         "origin",
-        &[],
+        None,
         git::RemoteCallbacks::default(),
         &git_settings,
     )
@@ -1105,7 +1105,7 @@ fn test_fetch_prune_deleted_ref() {
         tx.mut_repo(),
         &test_data.git_repo,
         "origin",
-        &[],
+        None,
         git::RemoteCallbacks::default(),
         &git_settings,
     )
@@ -1126,7 +1126,7 @@ fn test_fetch_no_default_branch() {
         tx.mut_repo(),
         &test_data.git_repo,
         "origin",
-        &[],
+        None,
         git::RemoteCallbacks::default(),
         &git_settings,
     )
@@ -1149,7 +1149,7 @@ fn test_fetch_no_default_branch() {
         tx.mut_repo(),
         &test_data.git_repo,
         "origin",
-        &[],
+        None,
         git::RemoteCallbacks::default(),
         &git_settings,
     )
@@ -1169,7 +1169,7 @@ fn test_fetch_no_such_remote() {
         tx.mut_repo(),
         &test_data.git_repo,
         "invalid-remote",
-        &[],
+        None,
         git::RemoteCallbacks::default(),
         &git_settings,
     );

src/commands/git.rs

@@ -302,7 +302,7 @@ fn cmd_git_fetch(
                 tx.mut_repo(),
                 &git_repo,
                 &remote,
-                &branches,
+                (!branches.is_empty()).then(|| &*branches),
                 cb,
                 &command.settings().git_settings(),
             )
@@ -430,7 +430,7 @@ fn do_git_clone(
             fetch_tx.mut_repo(),
             &git_repo,
             remote_name,
-            &[],
+            None,
             cb,
             &command.settings().git_settings(),
         )