git: use &[&str] instead of &[String]

Using &[String] forces the caller to materalize owned strings if they
have only references, which is costly. Using &[&str] makes it cheap
if the caller owns strings as well.

lib/src/git.rs

@@ -342,7 +342,7 @@ pub fn fetch(
     mut_repo: &mut MutableRepo,
     git_repo: &git2::Repository,
     remote_name: &str,
-    globs: &[String],
+    globs: &[&str],
     callbacks: RemoteCallbacks<'_>,
     git_settings: &GitSettings,
 ) -> Result<Option<String>, GitFetchError> {

src/commands/git.rs

@@ -295,13 +295,14 @@ fn cmd_git_fetch(
         "fetch from git remote(s) {}",
         remotes.iter().join(",")
     ));
+    let branches = args.branch.iter().map(|b| b.as_str()).collect_vec();
     for remote in remotes {
         with_remote_callbacks(ui, |cb| {
             git::fetch(
                 tx.mut_repo(),
                 &git_repo,
                 &remote,
-                &args.branch,
+                &branches,
                 cb,
                 &command.settings().git_settings(),
             )