patch 8.1.1731: command line history not read from viminfo on startup

Problem:    Command line history not read from viminfo on startup.
Solution:   Get history length after initializing it.

src/testdir/test_viminfo.vim

@@ -1,6 +1,12 @@
 " Test for reading and writing .viminfo
 
 function Test_viminfo_read_and_write()
+  " First clear 'history', so that "hislen" is zero.  Then set it again,
+  " simulating Vim starting up.
+  set history=0
+  wviminfo Xviminfo
+  set history=1000
+
   call histdel(':')
   let lines = [
 	\ '# comment line',

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1731,
 /**/
     1730,
 /**/

src/viminfo.c

@@ -212,9 +212,10 @@ prepare_viminfo_history(int asklen, int writing)
     int	    num;
     int	    type;
     int	    len;
-    int	    hislen = get_hislen();
+    int	    hislen;
 
     init_history();
+    hislen = get_hislen();
     viminfo_add_at_front = (asklen != 0 && !writing);
     if (asklen > hislen)
 	asklen = hislen;