Add ITERM_SESSION_ID environment variable.

PTYSession.m

@@ -488,8 +488,16 @@ static NSString* SESSION_ARRANGEMENT_WORKING_DIRECTORY = @"Working Directory";
         [env setObject:[self encodingName] forKey:@"LC_CTYPE"];
     }
-    if ([env objectForKey:PWD_ENVNAME] == nil)
+    if ([env objectForKey:PWD_ENVNAME] == nil) {
         [env setObject:[PWD_ENVVALUE stringByExpandingTildeInPath] forKey:PWD_ENVNAME];
+    }
+
+    PseudoTerminal *pty = [tab_ realParentWindow];
+    NSString *itermId = [NSString stringWithFormat:@"w%dt%dp%d",
+                         [pty number],
+                         [tab_ objectCount] - 1,
+                         [tab_ indexOfSession:self]];
+    [env setObject:itermId forKey:@"ITERM_SESSION_ID"];
     [SHELL launchWithPath:path
                 arguments:argv

PTYTab.h

@@ -104,6 +104,7 @@ static const int MIN_SESSION_COLUMNS = 2;
 - (void)setTabViewItem:(NSTabViewItem *)theTabViewItem;
 - (void)previousSession;
 - (void)nextSession;
+- (int)indexOfSession:(PTYSession*)session;
 - (void)setLockedSession:(PTYSession*)lockedSession;
 - (PTYSession*)activeSession;

PTYTab.m

@@ -404,6 +404,11 @@ static const BOOL USE_THIN_SPLITTERS = YES;
     }
 }
+- (int)indexOfSession:(PTYSession*)session
+{
+    return [[self sessions] indexOfObject:session];
+}
+
 - (id<WindowControllerInterface>)parentWindow
 {
     return parentWindow_;