This feature is intended to speed up git operations for repos
that have large working directories.
It enables git to work together with a file system monitor (see
the "fsmonitor-watchman" section of githooks(5)) that can inform
it as to what files have been modified. This enables git to avoid
having to lstat() every file to find modified files.
When used in conjunction with the untracked cache, it can further
improve performance by avoiding the cost of scanning the entire
working directory looking for new files.
If you want to enable (or disable) this feature, it is easier to
use the core.fsmonitor configuration variable (see git-config(1))
than using the --fsmonitor option to git update-index in each
repository, especially if you want to do so across all
repositories you use, because you can set the configuration
variable in your $HOME/.gitconfig just once and have it affect
all repositories you touch.
When the core.fsmonitor configuration variable is changed, the
file system monitor is added to or removed from the index the
next time a command reads the index. When --[no-]fsmonitor are
used, the file system monitor is immediately added to or removed
from the index.
