Allow opencode-pty to build against a vendored Ghostty source tree.

Upstream's build script requires GHOSTTY_SOURCE_DIR to be a Git checkout whose
HEAD matches ghostty-revision.  The port vendors Ghostty as a checksum-pinned
tarball, so skip the Git revision assertions when there is no repository to
compare against.

--- a/build.rs
+++ b/build.rs
@@ -54,32 +54,33 @@
 
     // The checked-in declarations must match the native headers exactly.
     // Local overrides are useful for builds, but not for silently changing ABI.
-    let head = Command::new("git")
-        .arg("-C")
-        .arg(&source)
-        .args(["rev-parse", "HEAD"])
-        .output()
-        .expect("read Ghostty revision");
-    assert!(
-        head.status.success(),
-        "GHOSTTY_SOURCE_DIR must be a Git checkout"
-    );
-    assert_eq!(
-        String::from_utf8(head.stdout)
-            .expect("Ghostty revision UTF-8")
-            .trim(),
-        revision,
-        "Ghostty headers and bindings are pinned together; regenerate before changing the revision"
-    );
-    run(Command::new("git").arg("-C").arg(&source).args([
-        "-c",
-        "core.longpaths=true",
-        "diff",
-        "--quiet",
-        "HEAD",
-        "--",
-        "include/ghostty",
-    ]));
+    // A vendored source tree (e.g. a checksum-pinned tarball, as used by the
+    // FreeBSD port) is not a Git checkout, so the revision assertions are
+    // skipped when there is no repository to compare against.
+    if source.join(".git").exists() {
+        let head = Command::new("git")
+            .arg("-C")
+            .arg(&source)
+            .args(["rev-parse", "HEAD"])
+            .output()
+            .expect("read Ghostty revision");
+        assert_eq!(
+            String::from_utf8(head.stdout)
+                .expect("Ghostty revision UTF-8")
+                .trim(),
+            revision,
+            "Ghostty headers and bindings are pinned together; regenerate before changing the revision"
+        );
+        run(Command::new("git").arg("-C").arg(&source).args([
+            "-c",
+            "core.longpaths=true",
+            "diff",
+            "--quiet",
+            "HEAD",
+            "--",
+            "include/ghostty",
+        ]));
+    }
 
     let target = env::var("TARGET").expect("TARGET");
     let install = out.join("ghostty-install");