Fix git hash retrieval during Nix builds

- Skip git operations when :nix-build feature is set
- Run git command in repository root instead of .git/ directory
- Add error handling to prevent build failures when git command fails

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: cxxxr

src/version.lisp

@@ -2,18 +2,23 @@
 
 (defun get-git-hash ()
   "Return lem's git hash."
+  ;; Skip git operations during Nix build
+  #-nix-build
   (let ((path (asdf:system-relative-pathname :lem ".git/")))
     (when (uiop:directory-exists-p path)
-      (uiop:with-current-directory (path)
-        (string-trim
-         (list #\Newline #\Space)
-	 #+sbcl
-         (with-output-to-string (stream)
-           (uiop:run-program "git rev-parse --short HEAD"
-                             :output stream))
-	 #-sbcl
-	 ""
-	 )))))
+      (let ((repo-root (uiop:pathname-directory-pathname path)))
+        (uiop:with-current-directory (repo-root)
+          (handler-case
+              (string-trim
+               (list #\Newline #\Space)
+               #+sbcl
+               (with-output-to-string (stream)
+                 (uiop:run-program "git rev-parse --short HEAD"
+                                   :output stream
+                                   :ignore-error-status nil))
+               #-sbcl
+               "")
+            (error () nil)))))))
 
 (defvar *git-revision* (get-git-hash)
   "Stores lem's git revision; this is treated as a cache.")