Improve .gitignore patterns (#3184)

* 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.

.gitignore

@@ -15,7 +15,6 @@
 *.Rcheck
 *.rds
 *.tar.gz
-#*txt*
 *conf
 *buffer
 *model
@@ -47,13 +46,12 @@ Debug
 *.cpage.col
 *.cpage
 *.Rproj
-./xgboost
 ./xgboost.mpi
 ./xgboost.mock
 #.Rbuildignore
 R-package.Rproj
 *.cache*
-#java
+# java
 java/xgboost4j/target
 java/xgboost4j/tmp
 java/xgboost4j-demo/target
@@ -68,10 +66,9 @@ nb-configuration*
 .settings/
 build
 config.mk
-xgboost
+/xgboost
 *.data
 build_plugin
-dmlc-core
 .idea
 recommonmark/
 tags