Merge branch 'bp/untracked-cache-noflush'

Writing out the index file when the only thing that changed in it is the untracked cache information is often wasteful, and this has been optimized out. * bp/untracked-cache-noflush: untracked cache: use git_env_bool() not getenv() for customization dir.c: don't flag the index as dirty for changes to the untracked cache