Add Documentation/perf-stat.txt and Documentation/perf-top.txt.

The template that was used for it: Documentation/git-add.txt from Git.

Fix up small bugs to make these help texts show up both in the 'perf'
common-command summary output screen, and on the individual help screens.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Lets use the Git framework of built-in commands.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

First very raw version at having a central 'perf' command and
a list of subcommands:

  perf top
  perf stat
  perf record
  perf report
  ...

This is done by picking up Git's collection of utility functions,
and hacking them to build fine in this new environment.
Signed-off-by: NIngo Molnar <mingo@elte.hu>