Disable Git panel button to open commit editor in certain cases (#26000)

Also:

- Internally renames a bit of code to make it easy to identify between
when we are disabling the buttons that open and close the modal editor
(in Git Panel and Project Diff) vs when we are disabling the commit
buttons (in Git Panel and Git commit editor modal).
- Deletes some unused code.

Release Notes:

- Unified disabling / enabling the button to open the Git commit editor
modal in the Git panel with the Project Diff commit button.
- Unified disabling / enabling the commit buttons, for the same cases,
between the Git panel and Git commit editor modal.

crates/git_ui/src/git_panel.rs

@@ -1917,7 +1917,7 @@ impl GitPanel {
         })
     }
 
-    pub fn can_commit(&self) -> bool {
+    pub fn can_open_commit_editor(&self) -> bool {
         (self.has_staged_changes() || self.has_tracked_changes()) && !self.has_unstaged_conflicts()
     }
 
@@ -2000,6 +2000,7 @@ impl GitPanel {
         let panel_editor_style = panel_editor_style(true, window, cx);
 
         if let Some(active_repo) = active_repository {
+            let can_open_commit_editor = self.can_open_commit_editor();
             let (can_commit, tooltip) = self.configure_commit_button(cx);
 
             let enable_coauthors = self.render_co_authors(cx);
@@ -2092,6 +2093,7 @@ impl GitPanel {
                                         .icon_size(IconSize::Small)
                                         .style(ButtonStyle::Transparent)
                                         .width(expand_button_size.into())
+                                        .disabled(!can_open_commit_editor)
                                         .on_click(cx.listener({
                                             move |_, _, window, cx| {
                                                 window.dispatch_action(