Make repo and branch popovers extend up from their trigger buttons (#26950)

Previously, when clicking on the branch, the popover would obscure the
button you just clicked, which was awkward.

Release Notes:

- Improved the placement of the repo and branch picker popovers in the
git panel.
- Added a 'SelectRepo' action that opens the repository selector in a
modal.

crates/zed_actions/src/lib.rs

@@ -116,7 +116,7 @@ pub mod workspace {
 pub mod git {
     use gpui::{action_with_deprecated_aliases, actions};
 
-    actions!(git, [CheckoutBranch, Switch]);
+    actions!(git, [CheckoutBranch, Switch, SelectRepo]);
     action_with_deprecated_aliases!(git, Branch, ["branches::OpenRecent"]);
 }