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Making git “forget” about a file that was tracked but is now in .gitignore:
.gitignore will prevent untracked files from being added (without an add -f) to the set of files tracked by git, however git will continue to track any files that are already being tracked.

To stop tracking a file you need to remove it from the index. This can be achieved with this command.
Files:
git rm --cached <file>
The removal of the file from the head revision will happen on the next commit.

Directory:
git rm -r --cached <folder>