[1.8.0] use 'stage' term consistently

Proposal:

Avoid the terms 'cache' and 'index' in favor of 'stage'.

Advantages:

The term 'stage' is more intuitive for newcomers which are more familiar with English than with git, and it seems to be a straightforward mental notion for people from different mother tongues.

It is so intuitive that it is used already in a lot online documentation, and the people that do teach git professionally use this term.

Risks:

People might be accustomed to the current options, and might take some time to get used to the new term. Scripts might be relying on the current options.

There's also the possibility that a lot of people prefer the terms 'cache' and 'index', but from the countless discussions on this subject, that seems to be rather unlikely.

Migration plan:

Follow a typical obsolete/deprecate process; for a period of time warn that the options are obsolete and shouldn't be used, in case there's a lot of people against this, this period would allow for them to shout; then remove them.

Rationale:

First of all, this discussion _always_ keeps coming back, so its clear something needs to be done, and in the last big discussion the consensus was that 'stage' was the best option. In summary:

cache: a 'cache' is a place for easier access; a squirrel caches nuts so it doesn't have to go looking for them in the future when it might be much more difficult. Git porcelain is not using the staging area for easier future access; it's not a cache.

index: an 'index' is a guide of pointers to something else; a book index has a list of entries so the reader can locate information easily without having to go through the whole book. Git porcelain is not using the staging area to find out entries quicker; it's not an index.

stage: a 'stage' is a special area designated for convenience in order for some activity to take place; an orator would prepare a stage in order for her speak to be successful, otherwise many people might not be able to hear, or see her. Git porcelain is using the staging area precisly as a special area to be separated from the working directory for convenience.

The term 'stage' is a good noun itself, but also 'staging area', it has a good verb; 'to stage', and a nice past-participle; 'staged'.