git_ui: Design Polish (#26361)

Polish PR

- [ ] Horizontal scrollbar for git panel
- [ ] Allow shift clicking a checkbox in any section to stage the whole
section
- [ ] Clean up design of no changes/pending push state in panel
- [x] Ensure checkbox placeholder dot is centered in the checkbox
- [x] Improve spacing between elements in git panel entries
- [x] Update git branch icon to match branch selector text when disabled
- [x] Truncate last commit message less aggressively in panel
- [x] Clean up new panel header design
- [x] Remove `_background` version control keys (backgrounds are derived
from the foreground colors)

### Previous message truncation:

Before:

![CleanShot 2025-03-10 at 11 54
32@2x](https://github.com/user-attachments/assets/46b18f66-bb5c-435e-a0da-6cc931bd8a15)

After:

![CleanShot 2025-03-10 at 11 55
24@2x](https://github.com/user-attachments/assets/fcf688c7-b949-41a2-a7b8-1a198eb7fa4a)

### Make branch icon match when menu is disabled

Before:

![CleanShot 2025-03-10 at 12 02
14@2x](https://github.com/user-attachments/assets/1990f4b3-c2f0-4e02-89ad-211aaebb3821)

After:

![CleanShot 2025-03-10 at 12 02
53@2x](https://github.com/user-attachments/assets/9b1caf65-c48f-44c9-924b-484892fb543f)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

---------

Co-authored-by: Cole Miller <cole@zed.dev>
Co-authored-by: Cole Miller <m@cole-miller.net>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/theme/src/styles/colors.rs

@@ -246,22 +246,14 @@ pub struct ThemeColors {
 
     /// Represents an added entry or hunk in vcs, like git.
     pub version_control_added: Hsla,
-    /// Represents the line background of an added entry or hunk in vcs, like git.
-    pub version_control_added_background: Hsla,
     /// Represents a deleted entry in version control systems.
     pub version_control_deleted: Hsla,
-    /// Represents the background color for deleted entries in version control systems.
-    pub version_control_deleted_background: Hsla,
     /// Represents a modified entry in version control systems.
     pub version_control_modified: Hsla,
-    /// Represents the background color for modified entries in version control systems.
-    pub version_control_modified_background: Hsla,
     /// Represents a renamed entry in version control systems.
     pub version_control_renamed: Hsla,
     /// Represents a conflicting entry in version control systems.
     pub version_control_conflict: Hsla,
-    /// Represents the background color for conflicting entries in version control systems.
-    pub version_control_conflict_background: Hsla,
     /// Represents an ignored entry in version control systems.
     pub version_control_ignored: Hsla,
 }
@@ -366,14 +358,10 @@ pub enum ThemeColorField {
     TerminalAnsiDimWhite,
     LinkTextHover,
     VersionControlAdded,
-    VersionControlAddedBackground,
     VersionControlDeleted,
-    VersionControlDeletedBackground,
     VersionControlModified,
-    VersionControlModifiedBackground,
     VersionControlRenamed,
     VersionControlConflict,
-    VersionControlConflictBackground,
     VersionControlIgnored,
 }
 
@@ -485,20 +473,10 @@ impl ThemeColors {
             ThemeColorField::TerminalAnsiDimWhite => self.terminal_ansi_dim_white,
             ThemeColorField::LinkTextHover => self.link_text_hover,
             ThemeColorField::VersionControlAdded => self.version_control_added,
-            ThemeColorField::VersionControlAddedBackground => self.version_control_added_background,
             ThemeColorField::VersionControlDeleted => self.version_control_deleted,
-            ThemeColorField::VersionControlDeletedBackground => {
-                self.version_control_deleted_background
-            }
             ThemeColorField::VersionControlModified => self.version_control_modified,
-            ThemeColorField::VersionControlModifiedBackground => {
-                self.version_control_modified_background
-            }
             ThemeColorField::VersionControlRenamed => self.version_control_renamed,
             ThemeColorField::VersionControlConflict => self.version_control_conflict,
-            ThemeColorField::VersionControlConflictBackground => {
-                self.version_control_conflict_background
-            }
             ThemeColorField::VersionControlIgnored => self.version_control_ignored,
         }
     }