BUG: groupby.cummin/cummax nans inf values for int64
 - [x] closes #15109 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

I migrated the `accum` portions of the code into cython and attempted to implement your suggested method to fix this issue @jreback. Let me know if this is along the right track. 

linting issue showing up & small comment, but otherwise  lgtm. ping when pushed and green. ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15205?src=pr) is 85.97% (diff: 100%)


> No coverage report found for **master** at 64d7670.

> Powered by [Codecov](https://codecov.io?src=pr). Last update [64d7670...b390fdb](https://codecov.io/gh/pandas-dev/pandas/compare/64d7670d99a11ec4e263da73f2bb1335cb4290d3...b390fdbe8bb2d89d0a687efe6bb5a2886d9fe499?src=pr) Ping all green.  thanks @mroeschke 