increase default scrollback limit

[Karl-Philipp Richter]

There's a really high change that (verbose) commands exceed 8192 lines out output and a very small chance that a high multiple (e.g. 8192*8192=67108864) causes any disadvantage. I think that setting a limit is already problematic - imagine running `btrfs check` for some hours and figuring out that most of you output is lost (might even be a problem with a scrollback limit of 67108864). Even a `~/.bash_history` of 50 MB shouldn't be a problem regarding average HDD size, btrfs compression and the ability to set `HISTSIZE` and `HISTFILESIZE`.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: gnome-terminal 3.14.2-0ubuntu2
ProcVersionSignature: Ubuntu 3.19.0-10.10-generic 3.19.2
Uname: Linux 3.19.0-10-generic x86_64
ApportVersion: 2.16.2-0ubuntu4
Architecture: amd64
CasperVersion: 1.356
CurrentDesktop: Unity
Date: Thu Apr 2 13:39:10 2015
LiveMediaBuild: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-terminal
UpgradeStatus: No upgrade log present (probably fresh install)

[Egmont Koblinger (egmont-gmail)]

Beware that:

- The scrollback contents need to reside somewhere. Currently it's on disk (compressed as of 0.40, but still). We shouldn't surprise users by filling up their disks.

- Content rewrapping on resize starts to become noticably slow at around 100.000 lines of scrollback.

IMO scrollback buffer is a conveniece feature, not something you should heavily rely on to be present under all circumstances. If you're running a command that you expect to produce lot of output and you do need the whole output, you should redirect that to a file.

That being said, you can increase it for yourself even to infinite and deal with the drawbacks. Given these drawbacks, I don't think the default should be bigger than 10.000 - 50.000-ish.