util: extract stub function to get mtime with second accuracy
This function is trivial but will need a long comment why it can't use
st.st_mtime. See the next patch for details.
scmutil: add a way for a repo's wlock to be inherited by a subprocess
This is part of a series that will allow locks to be inherited by subprocesses
in limited circumstances.
In the future, we'll call this for custom merge drivers.