Add multiple commit validator script

When we have multiple commits to be pushed, it is hard
to know if any of the lower commits other than HEAD commit
breaks any changes individually. This script can be used to
test a number of lower commits by running a test command on
each commit to ensure the sanity of commits.

Eg.

$ git-validate.sh 7 "make check"
verifying 70e7fea~7 ...OK
verifying 70e7fea~6 ...OK
verifying 70e7fea~5 ...OK
verifying 70e7fea~4 ...OK
verifying 70e7fea~3 ...OK
verifying 70e7fea~2 ...OK
verifying 70e7fea~1 ...OK
verifying 70e7fea~0 ...OK