patch 8.1.1823: command line history code is spread out

Problem:    Command line history code is spread out.
Solution:   Put the code in a new file. (Yegappan Lakshmanan, closes #4779)
            Also graduate the +cmdline_hist feature.
diff --git a/src/Make_morph.mak b/src/Make_morph.mak
index f75afd3..7616ce2 100644
--- a/src/Make_morph.mak
+++ b/src/Make_morph.mak
@@ -30,6 +30,7 @@
 	buffer.c						\
 	change.c						\
 	charset.c						\
+	cmdhist.c						\
 	crypt.c							\
 	crypt_zip.c						\
 	debugger.c						\