Add an advanced pref to keep CSI 3 J from erasing the scrollback buffer.

sources/VT100Screen.m

@@ -3301,7 +3301,9 @@ static NSString *const kInilineFileInset = @"inset";  // NSValue of NSEdgeInsets
 }
 - (void)terminalClearScrollbackBuffer {
-    [self clearScrollbackBuffer];
+    if (![iTermAdvancedSettingsModel preventEscapeSequenceFromClearingHistory]) {
+        [self clearScrollbackBuffer];
+    }
 }
 - (void)terminalClearBuffer {

sources/iTermAdvancedSettingsModel.h

@@ -189,5 +189,6 @@
 + (BOOL)disablePasswordManagerAnimations;
 + (BOOL)focusNewSplitPaneWithFocusFollowsMouse;
 + (NSString *)viewManPageCommand;
++ (BOOL)preventEscapeSequenceFromClearingHistory;
 @end

sources/iTermAdvancedSettingsModel.m

@@ -130,6 +130,7 @@ DEFINE_BOOL(useColorfgbgFallback, YES, @"Terminal: Use fallback for COLORFGBG if
 DEFINE_BOOL(zeroWidthSpaceAdvancesCursor, YES, @"Terminal: Zero-Width Space (U+200B) advances cursor?\nWhile a zero-width space should not advance the cursor per the Unicode spec, both Terminal.app and Konsole do this, and Weechat depends on it. You must restart iTerm2 after changing this setting.");
 DEFINE_BOOL(fullHeightCursor, NO, @"Terminal: Cursor occupies line spacing area.\nIf lines have more than 100% vertical spacing and this setting is enabled the bottom of the cursor will be aligned to the bottom of the spacing area.");
 DEFINE_FLOAT(underlineCursorOffset, 0, @"Terminal: Vertical offset for underline cursor.\nPositive values move it up, negative values move it down.");
+DEFINE_BOOL(preventEscapeSequenceFromClearingHistory, NO, @"Terminal: Prevent CSI 3 J from clearing scrollback history?\nThis is also known as thethe terminfo E3 capability.");
 #pragma mark Hotkey
 DEFINE_FLOAT(hotkeyTermAnimationDuration, 0.25, @"Hotkey: Duration in seconds of the hotkey window animation.\nWarning: reducing this value may cause problems if you have multiple displays.");