gitignore: only ignore root Debug directory

On case insensitive platforms, the rule catches the debug module under
npm and eslint and a source directory in next versions of V8.
Do the same with the Release directory for consistency.

Ref: #2286 (comment)
Ref: #2688 (comment)
PR-URL: #3144
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Roman Reiss <[email protected]>

Author: targos

.gitignore

@@ -23,9 +23,8 @@ icu_config.gypi
 /out
 
 # various stuff that VC++ produces/uses
-Debug/
-!node_modules/debug/
-Release/
+/Debug/
+/Release/
 !doc/blog/**
 *.sln
 !nodemsi.sln