Increase go-git cache size

[jfontan]

Right now there is no way of changing the default cache size in go-git and its size too small (96 MiB). I've been doing tests changing this value and its performance improved a lot.

Repositories: linux (2013), numpy, tensorflow
Number of rows: 395709
Query: SELECT count(*) FROM commits c NATURAL JOIN ref_commits r WHERE r.ref_name = 'HEAD';

Default cache: 1 row in set (54 min 22.17 sec)
Cache size * 8: 1 row in set (20 min 43.69 sec)

Memory consumption is also not too big. gitbase used 1.3 GiB in this query.

We should add an option to go-git Open to select cache size.

[eiso]

@jfontan how does this scale with increased memory in relation to repo size? The reason I am asking is that I much rather have a significantly faster response and have a customer use 100GB of ram for a query. High memory instances are cheap these days.

[jfontan]

@eiso, maybe it could heuristically assign a size but the fist step will be adding an option to change it.

[kuba--]

...or maybe we can start from something like 20% of RAM as default for gitbase