git: Add option to branch from default branch in branch picker (#34663)

Closes #33700

The option shows up as an icon that appears on entries that would create
a new branch. You can also branch from the default by secondary
confirming, which the icon has a tooltip for as well.

We based the default branch on the results from this command: `git
symbolic-ref refs/remotes/upstream/HEAD` and fallback to `git
symbolic-ref refs/remotes/origin/HEAD`

Release Notes:

- Add option to create a branch from a default branch in git branch
picker

---------

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

crates/proto/src/proto.rs

@@ -315,7 +315,9 @@ messages!(
     (LogToDebugConsole, Background),
     (GetDocumentDiagnostics, Background),
     (GetDocumentDiagnosticsResponse, Background),
-    (PullWorkspaceDiagnostics, Background)
+    (PullWorkspaceDiagnostics, Background),
+    (GetDefaultBranch, Background),
+    (GetDefaultBranchResponse, Background),
 );
 
 request_messages!(
@@ -483,7 +485,8 @@ request_messages!(
     (GetDebugAdapterBinary, DebugAdapterBinary),
     (RunDebugLocators, DebugRequest),
     (GetDocumentDiagnostics, GetDocumentDiagnosticsResponse),
-    (PullWorkspaceDiagnostics, Ack)
+    (PullWorkspaceDiagnostics, Ack),
+    (GetDefaultBranch, GetDefaultBranchResponse),
 );
 
 entity_messages!(
@@ -615,7 +618,8 @@ entity_messages!(
     GetDebugAdapterBinary,
     LogToDebugConsole,
     GetDocumentDiagnostics,
-    PullWorkspaceDiagnostics
+    PullWorkspaceDiagnostics,
+    GetDefaultBranch
 );
 
 entity_messages!(