Finish `gitlog` implementation

[patrickdevivo]

Currently, the gitlog implementation has some bugs (can't handle certain repos where there's output we haven't accounted for). We should harden this:

• Use a custom formatter that's more predictable than https://github.com/mergestat/gitutils/blob/main/gitlog/gitlog.go#L264 so that we can define exactly the fields we care about parsing
• Implement remaining options that are relevant to us