patch 8.1.0615: get_tv function names are not consistent

Problem:    Get_tv function names are not consistent.
Solution:   Rename to tv_get.
diff --git a/src/edit.c b/src/edit.c
index c235976..c6b0619 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -4363,7 +4363,7 @@
     }
     else
     {
-	word = get_tv_string_chk(tv);
+	word = tv_get_string_chk(tv);
 	vim_memset(cptext, 0, sizeof(cptext));
     }
     if (word == NULL || (!aempty && *word == NUL))