Merge pull request #13971 from edx/ned/make-clean

A perhaps too-baroque way to clean files but also keep some.

.gitignore

@@ -3,9 +3,13 @@
 
 ### Files private to developers
 
+# Files that should be git-ignored, but are hand-edited or otherwise valued,
+# and so should not be destroyed by "make clean".
+# start-noclean
 requirements/private.txt
 lms/envs/private.py
 cms/envs/private.py
+# end-noclean
 
 ### Python artifacts
 *.pyc

Makefile

+# Do things in edx-platform
+
+clean:
+	# Remove all the git-ignored stuff, but save and restore things marked
+	# by start-noclean/end-noclean.
+	sed -n -e '/start-noclean/,/end-noclean/p' < .gitignore > /tmp/private-files
+	tar cf /tmp/private.tar `git ls-files --exclude-from=/tmp/private-files --ignored --others`
+	git clean -fdX
+	tar xf /tmp/private.tar