In a tree in which I previously ran "make check" in contrib/pg_upgrade:
$ make -s distclean
$ git status
# On branch master
# Untracked files:
# (use "git add <file>..." to include in what will be committed)
#
# contrib/pg_upgrade/pg_upgrade_dump_1.log
# contrib/pg_upgrade/pg_upgrade_dump_12912.log
# contrib/pg_upgrade/pg_upgrade_dump_16384.log
nothing added to commit but untracked files present (use "git add" to track)
Not sure how long this has been happening.
regards, tom lane