git: allow both --branch/--bookmark for git fetch/push

`jj git push` has a `--bookmark` argument, which takes a list of
bookmarks to push to the Git remote. They'll become branches on the
Git remote. `jj git fetch` has a `--branch` argument, which takes a
list of Git branches to fetch. They'll become bookmarks once
fetched. So the naming is consistent, but the reasoning is quite
subtle. Let's provide the other name as a hidden alias to help users
who get it wrong.

cli/src/commands/git/fetch.rs

@@ -42,7 +42,7 @@ pub struct GitFetchArgs {
     ///
     /// By default, the specified name matches exactly. Use `glob:` prefix to
     /// expand `*` as a glob. The other wildcard characters aren't supported.
-    #[arg(long, short, default_value = "glob:*", value_parser = StringPattern::parse)]
+    #[arg(long, short, alias="bookmark", default_value = "glob:*", value_parser = StringPattern::parse)]
     branch: Vec<StringPattern>,
     /// The remote to fetch from (only named remotes are supported, can be
     /// repeated)

cli/src/commands/git/push.rs

@@ -84,7 +84,7 @@ pub struct GitPushArgs {
     /// By default, the specified name matches exactly. Use `glob:` prefix to
     /// select bookmarks by wildcard pattern. For details, see
     /// https://martinvonz.github.io/jj/latest/revsets#string-patterns.
-    #[arg(long, short, value_parser = StringPattern::parse)]
+    #[arg(long, short, alias="branch", value_parser = StringPattern::parse)]
     bookmark: Vec<StringPattern>,
     /// Push all bookmarks (including deleted bookmarks)
     #[arg(long)]