Fix missing build/make/Makefile error

.ci/retrofit-worktree.sh

@@ -20,27 +20,33 @@ git config --global user.email "$GIT_AUTHOR_EMAIL"
 
 set -x
 
-# If actions/checkout downloaded our source tree using the GitHub REST API
-# instead of with git, we first make the source tree a repo.
+# If actions/checkout downloaded our source tree (in current directory) using the
+# GitHub REST API instead of with git, we first make the source tree a repo.
 if [ ! -d .git ]; then git init; fi
 
-# Commit and tag this state of the source tree "new". This is what we want to build and test.
+# Commit and tag this state of the source tree "new". The current directory
+# contains "new" source files with which we want to build and test.
 git add -A && git commit --quiet --allow-empty -m "new"
 git tag -f new
 
 # Our container image contains a source tree in $WORKTREE_DIRECTORY with a full build of Sage.
 # But $WORKTREE_DIRECTORY is not a git repository.
 # We make $WORKTREE_DIRECTORY a worktree whose index is at tag "new".
-# We then commit the current sources and set the tag "old". (This keeps all mtimes unchanged.)
-# Then we update worktree and index with "git checkout new".
-# (This keeps mtimes of unchanged files unchanged and mtimes of changed files newer than unchanged files.)
 if [ -L $WORKTREE_NAME ]; then
     rm -f $WORKTREE_NAME
 fi
 git worktree prune --verbose
 git worktree add --detach $WORKTREE_NAME
 rm -rf $WORKTREE_DIRECTORY/.git && mv $WORKTREE_NAME/.git $WORKTREE_DIRECTORY/
 rm -rf $WORKTREE_NAME && ln -s $WORKTREE_DIRECTORY $WORKTREE_NAME
-if [ ! -f $WORKTREE_NAME/.gitignore ]; then cp .gitignore $WORKTREE_NAME/; cat $WORKTREE_NAME/.gitignore; fi
-(cd $WORKTREE_NAME && git status && git add -A && git commit --quiet --allow-empty -m "old" -a && git tag -f old && git checkout -f new && git clean -fd && git status)
-(echo "CIDEBUG6:"; ls -l /sage/build/make/Makefile || cat $WORKTREE_NAME/.gitignore)
+
+# Copy "new" .gitignore (in current directory) to the worktree if there is not one already.
+if [ ! -f $WORKTREE_NAME/.gitignore ]; then cp .gitignore $WORKTREE_NAME/; fi
+# We then commit the worktree and set the tag "old". (This keeps all modification times unchanged.)
+(cd $WORKTREE_NAME && git add -A && git commit --quiet --allow-empty -m "old" && git tag -f old)
+# Then we update the worktree and index with "git checkout new". (This keeps modification times of "old" files
+# and makes modification times of "new" files newer, which triggers incremental build.)
+(cd $WORKTREE_NAME && git checkout -f new && \
+ # For unknown reason, "build/" directory is untracked by git. Hence we force
+ # it tracked, to avoid removal of "build/make/Makefile" by "git clean -fd"
+ git add build/ && git clean -fd)