git log -G'regex' is a very useful alternative to the classic
pickaxe.  Minimal patch to make it usable from gitk.

[zj: reword message]
[paulus@samba.org: reword droplist item]
Signed-off-by: NZbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: NPaul Mackerras <paulus@samba.org>