worktree: update renamed variable in comment

The comment above `add_head_info()` mentions "head_sha1", but it was
renamed to "head_oid" in 0f05154c70 ("worktree: convert struct worktree
to object_id", 2017-10-15). Update the comment.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/worktree.c b/worktree.c
index a98f77d..f5eb6e7 100644
--- a/worktree.c
+++ b/worktree.c
@@ -21,7 +21,7 @@
 }
 
 /**
- * Update head_sha1, head_ref and is_detached of the given worktree
+ * Update head_oid, head_ref and is_detached of the given worktree
  */
 static void add_head_info(struct worktree *wt)
 {