git: Amend (#28187)

Adds git amend support.

- [x] Turn existing commit button into split button
- [x] Clean up + Handle shortcuts/focus cases
- [x] Test remote

Release Notes:

- Added git amend support.

---------

Co-authored-by: Cole Miller <cole@zed.dev>

assets/keymaps/default-linux.json

@@ -782,6 +782,7 @@
       "shift-tab": "git_panel::FocusEditor",
       "escape": "git_panel::ToggleFocus",
       "ctrl-enter": "git::Commit",
+      "ctrl-shift-enter": "git::Amend",
       "alt-enter": "menu::SecondaryConfirm",
       "delete": ["git::RestoreFile", { "skip_prompt": false }],
       "backspace": ["git::RestoreFile", { "skip_prompt": false }],
@@ -790,12 +791,20 @@
       "ctrl-delete": ["git::RestoreFile", { "skip_prompt": false }]
     }
   },
+  {
+    "context": "GitPanel && CommitEditor",
+    "use_key_equivalents": true,
+    "bindings": {
+      "escape": "git::Cancel"
+    }
+  },
   {
     "context": "GitCommit > Editor",
     "bindings": {
       "escape": "menu::Cancel",
       "enter": "editor::Newline",
       "ctrl-enter": "git::Commit",
+      "ctrl-shift-enter": "git::Amend",
       "alt-l": "git::GenerateCommitMessage"
     }
   },
@@ -817,6 +826,7 @@
     "context": "GitDiff > Editor",
     "bindings": {
       "ctrl-enter": "git::Commit",
+      "ctrl-shift-enter": "git::Amend",
       "ctrl-space": "git::StageAll",
       "ctrl-shift-space": "git::UnstageAll"
     }
@@ -835,6 +845,7 @@
       "shift-tab": "git_panel::FocusChanges",
       "enter": "editor::Newline",
       "ctrl-enter": "git::Commit",
+      "ctrl-shift-enter": "git::Amend",
       "alt-up": "git_panel::FocusChanges",
       "alt-l": "git::GenerateCommitMessage"
     }