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/git/src/repository.rs

@@ -106,6 +106,7 @@ pub struct CommitSummary {
     pub subject: SharedString,
     /// This is a unix timestamp
     pub commit_timestamp: i64,
+    pub has_parent: bool,
 }
 
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
@@ -471,6 +472,7 @@ impl GitRepository for RealGitRepository {
         let fields = [
             "%(HEAD)",
             "%(objectname)",
+            "%(parent)",
             "%(refname)",
             "%(upstream)",
             "%(upstream:track)",
@@ -1126,6 +1128,7 @@ fn parse_branch_input(input: &str) -> Result<Vec<Branch>> {
         let mut fields = line.split('\x00');
         let is_current_branch = fields.next().context("no HEAD")? == "*";
         let head_sha: SharedString = fields.next().context("no objectname")?.to_string().into();
+        let parent_sha: SharedString = fields.next().context("no parent")?.to_string().into();
         let ref_name: SharedString = fields
             .next()
             .context("no refname")?
@@ -1149,6 +1152,7 @@ fn parse_branch_input(input: &str) -> Result<Vec<Branch>> {
                 sha: head_sha,
                 subject,
                 commit_timestamp: commiterdate,
+                has_parent: !parent_sha.is_empty(),
             }),
             upstream: if upstream_name.is_empty() {
                 None
@@ -1201,7 +1205,7 @@ fn parse_upstream_track(upstream_track: &str) -> Result<UpstreamTracking> {
 fn test_branches_parsing() {
     // suppress "help: octal escapes are not supported, `\0` is always null"
     #[allow(clippy::octal_escapes)]
-    let input = "*\0060964da10574cd9bf06463a53bf6e0769c5c45e\0refs/heads/zed-patches\0refs/remotes/origin/zed-patches\0\01733187470\0generated protobuf\n";
+    let input = "*\0060964da10574cd9bf06463a53bf6e0769c5c45e\0\0refs/heads/zed-patches\0refs/remotes/origin/zed-patches\0\01733187470\0generated protobuf\n";
     assert_eq!(
         parse_branch_input(&input).unwrap(),
         vec![Branch {
@@ -1218,6 +1222,7 @@ fn test_branches_parsing() {
                 sha: "060964da10574cd9bf06463a53bf6e0769c5c45e".into(),
                 subject: "generated protobuf".into(),
                 commit_timestamp: 1733187470,
+                has_parent: false,
             })
         }]
     )

crates/git_ui/src/git_panel.rs

@@ -2237,20 +2237,28 @@ index 1234567..abcdef0 100644
                         }),
                 )
                 .child(div().flex_1())
-                .child(
-                    panel_icon_button("undo", IconName::Undo)
-                        .icon_size(IconSize::Small)
-                        .icon_color(Color::Muted)
-                        .tooltip(Tooltip::for_action_title(
-                            if self.has_staged_changes() {
-                                "git reset HEAD^ --soft"
-                            } else {
-                                "git reset HEAD^"
-                            },
-                            &git::Uncommit,
-                        ))
-                        .on_click(cx.listener(|this, _, window, cx| this.uncommit(window, cx))),
-                ),
+                .when(commit.has_parent, |this| {
+                    let has_unstaged = self.has_unstaged_changes();
+                    this.child(
+                        panel_icon_button("undo", IconName::Undo)
+                            .icon_size(IconSize::Small)
+                            .icon_color(Color::Muted)
+                            .tooltip(move |window, cx| {
+                                Tooltip::with_meta(
+                                    "Uncommit",
+                                    Some(&git::Uncommit),
+                                    if has_unstaged {
+                                        "git reset HEAD^ --soft"
+                                    } else {
+                                        "git reset HEAD^"
+                                    },
+                                    window,
+                                    cx,
+                                )
+                            })
+                            .on_click(cx.listener(|this, _, window, cx| this.uncommit(window, cx))),
+                    )
+                }),
         )
     }
 
@@ -3559,6 +3567,7 @@ impl ComponentPreview for PanelRepoFooter {
                     sha: "abc123".into(),
                     subject: "Modify stuff".into(),
                     commit_timestamp: 1710932954,
+                    has_parent: true,
                 }),
             }
         }
@@ -3575,6 +3584,7 @@ impl ComponentPreview for PanelRepoFooter {
                     sha: "abc123".into(),
                     subject: "Modify stuff".into(),
                     commit_timestamp: 1710932954,
+                    has_parent: true,
                 }),
             }
         }

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,
             }
         }),
     }