Move most of the root package into a subcrate. (#1445)

This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).

crates/nu-cli/src/git.rs

@@ -0,0 +1,21 @@
+#![cfg(not(feature = "starship-prompt"))]
+
+use git2::{Repository, RepositoryOpenFlags};
+use std::ffi::OsString;
+
+pub fn current_branch() -> Option<String> {
+    let v: Vec<OsString> = vec![];
+    match Repository::open_ext(".", RepositoryOpenFlags::empty(), v) {
+        Ok(repo) => {
+            let r = repo.head();
+            match r {
+                Ok(r) => match r.shorthand() {
+                    Some(s) => Some(s.to_string()),
+                    None => None,
+                },
+                _ => None,
+            }
+        }
+        _ => None,
+    }
+}