Disable uncommit button for parentless commits (#25983)

Closes #25976

There's a couple states that this covers:
- upon `git init`, no footer is shown at all
- after 1 commit (or when on any parentless commit), the uncommit button
is ~disabled~ hidden
- otherwise commit button is shown

Also updated the button with "meta" tooltip showing human readable
description and git command.

Release Notes:

- N/A

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>

crates/worktree/src/worktree.rs

@@ -382,6 +382,7 @@ pub fn proto_to_branch(proto: &proto::Branch) -> git::repository::Branch {
                 sha: commit.sha.to_string().into(),
                 subject: commit.subject.to_string().into(),
                 commit_timestamp: commit.commit_timestamp,
+                has_parent: true,
             }
         }),
     }