Add Delete All Untracked Files to git til.

git/delete-all-untracked-files.md

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+# Delete All Untracked Files
+
+Git provides a command explicitly intended for cleaning up (read: removing)
+untracked files from a local copy of a repository.
+
+> git-clean - Remove untracked files from the working tree
+
+Git does want you to be explicit though and requires you to use the `-f`
+flag to force it (unless otherwise configured).
+
+Git also gives you fine-grained control with this command by defaulting to
+only deleting untracked files in the current directory. If you want
+directories of untracked files to be removed as well, you'll need the `-d`
+flag.
+
+So if you have a local repository full of files you'd like to get rid of,
+just:
+
+```bash
+$ git clean -f -d
+```
+
+or just:
+
+```bash
+$ git clean -fd
+```