patch 8.0.1712: terminal scrollback is not limited

Problem:    Terminal scrollback is not limited.
Solution:   Add the 'terminalscroll' option.
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index cc19e25..0820699 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 8.0.  Last change: 2018 Mar 13
+*options.txt*	For Vim version 8.0.  Last change: 2018 Apr 14
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -7933,6 +7933,16 @@
 	Note that the "cterm" attributes are still used, not the "gui" ones.
 	NOTE: This option is reset when 'compatible' is set.
 
+						*'terminalscroll'* *'tlsl'*
+'terminalscroll' 'tlsl'	number	(default 10000)
+			global
+			{not in Vi}
+			{not available when compiled without the
+			|+terminal| feature}
+	Number of scrollback lines to keep.  When going over this limit the
+	first 10% of the scrollback lines are deleted.  This is just to reduce
+	the memory usage.  See |Terminal-Normal|.
+
 						*'termkey'* *'tk'*
 'termkey' 'tk'		string	(default "")
 			local to window
diff --git a/runtime/doc/terminal.txt b/runtime/doc/terminal.txt
index 63217c2..23e210d 100644
--- a/runtime/doc/terminal.txt
+++ b/runtime/doc/terminal.txt
@@ -288,7 +288,7 @@
 
 
 Terminal-Job and Terminal-Normal mode ~
-							*Terminal-mode*
+						*Terminal-mode* *Terminal-Job*
 When the job is running the contents of the terminal is under control of the
 job.  That includes the cursor position.  Typed keys are sent to the job.
 The terminal contents can change at any time.  This is called Terminal-Job
@@ -301,7 +301,9 @@
 Terminal-Job mode is where |:tmap| mappings are applied. Keys sent by
 |term_sendkeys()| are not subject to tmap, but keys from |feedkeys()| are.
 
-							*E946*
+It is not possible to enter Insert mode from Terminal-Job mode.
+
+						*Terminal-Normal* *E946*
 In Terminal-Normal mode you can move the cursor around with the usual Vim
 commands, Visually mark text, yank text, etc.  But you cannot change the
 contents of the buffer.  The commands that would start insert mode, such as
@@ -312,7 +314,10 @@
 the job ends while in Terminal-Normal mode this changes to
 "(Terminal-finished)".
 
-It is not possible to enter Insert mode from Terminal-Job mode.
+When the job outputs lines in the terminal, such that the contents scrolls off
+the top, those lines are remembered and can be seen in Terminal-Normal mode.
+The number of lines is limited by the 'terminalscroll' option. When going over
+this limit, the first 10% of the scrolled lins are deleted and are lost.
 
 
 Cursor style ~
diff --git a/src/option.c b/src/option.c
index 5ef346c..9ebc511 100644
--- a/src/option.c
+++ b/src/option.c
@@ -2750,6 +2750,15 @@
 			    {(char_u *)FALSE, (char_u *)FALSE}
 #endif
 			    SCRIPTID_INIT},
+    {"terminalscroll", "tlsl", P_NUM|P_VI_DEF|P_VIM|P_RBUF,
+#ifdef FEAT_TERMINAL
+			    (char_u *)&p_tlsl, PV_NONE,
+			    {(char_u *)10000L, (char_u *)10000L}
+#else
+			    (char_u *)NULL, PV_NONE,
+			    {(char_u *)NULL, (char_u *)0L}
+#endif
+			    SCRIPTID_INIT},
     {"termkey", "tk",	    P_STRING|P_ALLOCED|P_RWIN|P_VI_DEF,
 #ifdef FEAT_TERMINAL
 			    (char_u *)VAR_WIN, PV_TK,
diff --git a/src/option.h b/src/option.h
index d2ee545..45f1a36 100644
--- a/src/option.h
+++ b/src/option.h
@@ -849,6 +849,9 @@
 #ifdef FEAT_ARABIC
 EXTERN int	p_tbidi;	/* 'termbidi' */
 #endif
+#ifdef FEAT_TERMINAL
+EXTERN long	p_tlsl;		/* 'terminalscroll' */
+#endif
 #ifdef FEAT_MBYTE
 EXTERN char_u	*p_tenc;	/* 'termencoding' */
 #endif
diff --git a/src/terminal.c b/src/terminal.c
index 09d4887..7162d68 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -40,8 +40,6 @@
  * TODO:
  * - Win32: Make terminal used for :!cmd in the GUI work better.  Allow for
  *   redirection.  Probably in call to channel_set_pipes().
- * - add an optional limit for the scrollback size.  When reaching it remove
- *   10% at the start.
  * - Copy text in the vterm to the Vim buffer once in a while, so that
  *   completion works.
  * - in GUI vertical split causes problems.  Cursor is flickering. (Hirohito
@@ -2518,7 +2516,27 @@
 {
     term_T	*term = (term_T *)user;
 
-    /* TODO: Limit the number of lines that are stored. */
+    /* If the number of lines that are stored goes over 'termscrollback' then
+     * delete the first 10%. */
+    if (term->tl_scrollback.ga_len > p_tlsl)
+    {
+	int	todo = p_tlsl / 10;
+	int	i;
+
+	curbuf = term->tl_buffer;
+	for (i = 0; i < todo; ++i)
+	{
+	    vim_free(((sb_line_T *)term->tl_scrollback.ga_data + i)->sb_cells);
+	    ml_delete(1, FALSE);
+	}
+	curbuf = curwin->w_buffer;
+
+	term->tl_scrollback.ga_len -= todo;
+	mch_memmove(term->tl_scrollback.ga_data,
+	    (sb_line_T *)term->tl_scrollback.ga_data + todo,
+	    sizeof(sb_line_T) * term->tl_scrollback.ga_len);
+    }
+
     if (ga_grow(&term->tl_scrollback, 1) == OK)
     {
 	cellattr_T	*p = NULL;
diff --git a/src/version.c b/src/version.c
index 255e24b..41b9721 100644
--- a/src/version.c
+++ b/src/version.c
@@ -763,6 +763,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1712,
+/**/
     1711,
 /**/
     1710,