Add 'git-rev-parse' helper script

Add 'git-rev-parse' helper script

It's an incredibly cheesy helper that changes human-readable revision
arguments into the git-rev-list argument format.

You can use it to do something like this:

	git-rev-list --pretty $(git-rev-parse --default HEAD "$@")

which is what git-log-script will become. Here git-rev-parse will
then allow you to use arguments like "v2.6.12-rc5.." or similar
human-readable ranges.

It's really quite stupid: "a..b" will be converted into "a" and "^b" if
"a" and "b" are valid object pointers.  And the "--default" case will be
used if nothing but flags have been seen, so that you can default to a
certain argument if there are no other ranges.