Remove calls to UDFs whose index is being used

[erizocosmico]

Right now, expression with used indexes are not being removed. They should be to avoid expensive calls to some UDFs.

They are not removed right now because it might clash with the squash joins rule. We need to implement this in a way that does not break that rule.

[erizocosmico]

Blocked until indexes are implemented in gitbase

[erizocosmico]

This would require implementing rule phases, because this would require the squash rule to be done before assigning indexes. Right now either you mess with inserting the rule in a certain position or append it to the end (which is what we're doing).

Once that is done, the squash would be done first and the indexes can be assigned to the topmost table of the squash only.

Then, we can always remove the expression that produces the index.

[ajnavarro]

Depends on #141

[erizocosmico]

I created src-d/gitbase#318 to keep track of the part that concerns gitbase. There are some details there worth taking into account with this task.

[erizocosmico]

Blocked again until further discussion, see src-d/gitbase#318

[ajnavarro]

This is not blocked anymore.

[erizocosmico]

How is this not blocked? We can't still do this because of the squashed tables.

[ajnavarro]

Right, misreading.