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commit: make commit acquire store lock before processing for consistency

If acquisition of wlock waits for another "hg commit" process to
release it, dirstate will refer newly committed revision after
acquisition of wlock.

At that time, '00changelog.i' on the filesystem contains this new
revision, but in-memory 'repo.changelog' doesn't, if it is cached
without store lock (slock) before updating by another "hg commit".

This makes validating parents at re-loading 'repo.dirstate' from
'.hg/dirstate' replace such new revision with 'nullid'. Then,
'localrepository.commit()' creates "orphan" revision (see issue4368
for detail).

a01d3d32b53a makes 'commands.commit()' acquire both wlock and slock
before processing to avoid this issue at "hg commit".

But similar issue can occur even after a01d3d32b53a, if 3rd party
extension does:

  - refer 'repo.changelog' outside wlock scope, and
  - invoke 'repo.commit()' directly (instead of 'commands.commit()')

This patch makes 'commit()' acquire slock before processing, to refer
recent changelog at validating parents of 'repo.dirstate'.

Mercurial
=========

Mercurial is a fast, easy to use, distributed revision control tool
for software developers.

Basic install:

 $ make            # see install targets
 $ make install    # do a system-wide install
 $ hg debuginstall # sanity-check setup
 $ hg              # see help

Running without installing:

 $ make local      # build for inplace usage
 $ ./hg --version  # should show the latest version

See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.