git_array_alloc: return objects of correct type

[boretrk]

Unlike other array macros git_array_alloc would return a void pointer when an object needed to be allocated.
By changing git_array_grow to only grow the array and make the actual allocation in the git_array_alloc macro the returned type will always be correct.

[ethomson]

wow this macro is getting unwieldly 😛

[boretrk]

Functionally it would be possible to move the first size-comparison into the the grow-function now, then it will be pretty close to the other macros.
But since inlining relies on compiler extensions for C89 I'd rather keep it where it is since that will prevent git_array_grow from being called most of the time when inlining doesn't work.

[ethomson]

Sorry, that was more a comment than a criticism. 😁 Thanks for tackling this, these are nice improvements.

[ethomson]

I think that this makes sense. I'm a little surprised that there's only the one user of git_array_grow.

[boretrk]

I think that this makes sense. I'm a little surprised that there's only the one user of git_array_grow.

Everyone calls git_array_alloc instead.
The allocation has to be made as a macro to be able to extract the item size from the array type.
Then git_array_grow is called as a generic allocator with the item size as an argument, but there is very little reason for any function to call git_array_grow directly.