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fsmonitor: hook up state-enter, state-leave signals

Keeping the codebase in sync with upstream:

Watchman 4.4 introduced an advanced settling feature that allows publishing
tools to notify subscribing tools of the boundaries for important filesystem
operations.

https://facebook.github.io/watchman/docs/cmd/subscribe.html#advanced-settling
has more information about how this feature works.

This diff connects a signal that we're calling `hg.update` to the mercurial
update function so that mercurial can indirectly notify tools (such as IDEs or
build machinery) when it is changing the working copy.  This will allow those
tools to pause their normal actions as the files are changing and defer them
until the end of the operation.

In addition to sending the enter/leave signals for the state, we are able to
publish useful metadata along the same channel.  In this case we are passing
the following pieces of information:

1. destination revision hash
2. An estimate of the distance between the current state and the target state
3. A success indicator.
4. Whether it is a partial update

The distance is estimate may be useful to tools that wish to change their
strategy after the update has complete.  For example, a large update may be
efficient to deal with by walking some internal state in the subscriber rather
than feeding every individual file notification through its normal (small)
delta mechanism.

We estimate the distance by comparing the repository revision number.  In some
cases we cannot come up with a number so we report 0.  This is ok; we're
offering this for informational purposes only and don't guarantee its accuracy.

The success indicator is only really meaningful when we generate the
state-leave notification; it indicates the overall success of the update.

#!/usr/bin/env python
#
# An example hgweb CGI script, edit as necessary
# See also https://mercurial-scm.org/wiki/PublishingRepositories

# Path to repo or hgweb config to serve (see 'hg help hgweb')
config = "/path/to/repo/or/config"

# Uncomment and adjust if Mercurial is not installed system-wide
# (consult "installed modules" path from 'hg debuginstall'):
#import sys; sys.path.insert(0, "/path/to/python/lib")

# Uncomment to send python tracebacks to the browser if an error occurs:
#import cgitb; cgitb.enable()

from mercurial import demandimport; demandimport.enable()
from mercurial.hgweb import hgweb, wsgicgi
application = hgweb(config)
wsgicgi.launch(application)