fix: repeat if sum creates dummy indices

[jodavies]

This fixes #615 , but not the other "mode" of sum (TYPESUMFIX), i.e. if we have something like sum $sumind,1,2,3,4;.

I am not sure yet how to fix things in that case: it is inside the call of Generator here

form/sources/proces.c

Line 3529 in d9a3230

if ( Generator(BHEAD termout,level) ) goto GenCall;

where we need to repeat the statements of the repeat block, right? The term we started with disappears. Setting RepCount before or after the call of Generator here doesn't work.

[coveralls]

coverage: 50.875% (+0.01%) from 50.861%
when pulling 8974af9 on jodavies:repeat-sum
into 8c9e36e on form-dev:master.

[jodavies]

Updated to add a test. I'll merge this, with the caveat that the sum $sumind,1,2,3,4; mode of sum still doesn't work. Fixing this can be investigated separately. There are anyway ways to "workaround" this in a FORM script.