Git context menu (#24844)

Adds the non-entry specific right click menu to the panel, and the
features contained therin:

* Stage all
* Discard Tracked Changes
* Trash Untracked Files

Also changes the naming from "Changes"/"New" to better match Git's
terminology (though not convinced on this, it was awkward to describe
"Discard Changes" without a way to distinguish between the changes and
the files containing them).

Release Notes:

- N/A

crates/proto/src/proto.rs

@@ -441,6 +441,7 @@ messages!(
     (InstallExtension, Background),
     (RegisterBufferWithLanguageServers, Background),
     (GitReset, Background),
+    (GitCheckoutFiles, Background),
     (GitShow, Background),
     (GitCommitDetails, Background),
     (SetIndexText, Background),
@@ -579,6 +580,7 @@ request_messages!(
     (RegisterBufferWithLanguageServers, Ack),
     (GitShow, GitCommitDetails),
     (GitReset, Ack),
+    (GitCheckoutFiles, Ack),
     (SetIndexText, Ack),
 );
 
@@ -674,6 +676,7 @@ entity_messages!(
     RegisterBufferWithLanguageServers,
     GitShow,
     GitReset,
+    GitCheckoutFiles,
     SetIndexText,
 );