* Adjust xgboost entries in .gitignore
They were overly broad. In particularly this was inconvenient when
working with tools such as fzf that use the .gitignore to decide what to
include. As written, we'd not look into /include/xgboost.
* Make cosmetic improvements to .gitignore
* Remove dmlc-core from .gitignore
This seems unnecessary and has the drawback that tools that use
.gitignore to know files to skip mean they won't look here, and being
able to inspect the submodule files with them is useful.
Will Storey