tracing: Add vim script to enable folding for function_graph traces

function_graph traces look like nested function calls, complete with
braces denoting the start and end of functions. function-graph-fold.vim
teaches vim how to fold these functions, to make it more convenient to
browse them.

To use, :source function-graph-fold.vim while viewing a function_graph
trace, or use "view -S function-graph-fold.vim some-trace" to load it
from the command-line together with a trace. You can then use the usual
vim fold commands, such as "za", to open and close nested functions.
While closed, a fold will show the total time taken for a call, as would
normally appear on the line with the closing brace. Folded functions
will not include finish_task_switch(), so folding should remain
relatively sane even through a context switch.

Note that this will almost certainly only work well with a single-CPU
trace (e.g. trace-cmd report --cpu 1). It also takes some time to run
(a few seconds for a large trace on my laptop). Nevertheless, I found
it very handy to get an overview of a trace and then drill down on
problematic calls.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
LKML-Reference: <20090806145701.GB7661@feather>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>