Add an option to checkout the tree at a given timestamp

Test: use the --timestamp option and check the version it generates.
      Also use --version=latest to check out the version it uses.

PRESUBMIT=passed
R=asharif
APPROVED=asharif
DELTA=49  (45 added, 2 deleted, 2 changed)
OCL=63008-p2
RCL=63033-p2
RDATE=2012/09/26 09:18:09


P4 change: 42793622
1 file changed
tree: da1459a0b8e749408b7842559e45c95e42f7eae2
  1. .git5_specs/
  2. v14/