fetchFromGitLab: force re-fetch when rev changes

[bbjubjub2494]

There are a few instances of fetchFromGitLab in the repo, for instance gtk-doc.src.

My primary goal when writing this was changing the output derivation following #11. This is doable with a lot less diff in fetchFromGitLab, but it's worth asking whether both fetchers should be different and why. I think a lot of the divergence comes from years of random commits rather than from design.

[Sorixelle]

I'd be inclined to rework both, and use what exists now as a base for a generic fetchFromGitForge that both fetchFromGitHub and fetchFromGitlab can be defined in terms of.

[bbjubjub2494]

I tried working on the refactor in a branch, but I am not comfortable writing such code for production because there seem to be too many small details in the original code like complex passthroughs and unsafe functions that I don't know the purpose of.
https://github.com/bbjubjub2494/auxolotl_core/tree/fetchfromgitforge

I would suggest we do the minimal diff thing for now