Cope with gecos being unset.

NEWS

@@ -1,5 +1,11 @@
 0.20.28	UNRELEASED
 
+ * Fix hook test on Mac OSX / Linux when dulwich is
+   not installed system-wide. (Jelmer Vernooij, #919)
+
+ * Cope with gecos being unset.
+   (Jelmer Vernooij, #917)
+
 0.20.27	2022-01-04
 
  * Allow adding files to repository in pre-commit hook.

dulwich/repo.py

@@ -146,7 +146,10 @@ def _get_default_identity() -> Tuple[str, str]:
         except KeyError:
             fullname = None
         else:
-            fullname = gecos.split(",")[0]
+            if gecos:
+                fullname = gecos.split(",")[0]
+            else:
+                fullname = None
     if not fullname:
         fullname = username
     email = os.environ.get("EMAIL")