workers: don't use backgroundfilecloser in threads
This disables background file closing when in not in main thread
Test Plan:
Ran pull, update, sparse commands and watched the closer threads created and destroyed in procexp.exe
ran test on CentOS. No tests broken compared to the base
Differential Revision: https://phab.mercurial-scm.org/D1457
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true