commit: use context manager with dirstateguard When I wrote 5ac845ca059a (commit: don't let failed commit with --addremove update dirstate (issue5645), 2017-07-31), Durham's 609606d21765 (rebase: use one dirstateguard for when using rebase.singletransaction, 2017-07-20) had not yet landed, so I had to write it in the old-fashioned way. Now that Durham's patch is in, we can simplify by using a context manager. Differential Revision: https://phab.mercurial-scm.org/D406

commit: move dirstateguard creation out of try-block This is just a simple refactoring to make the next patch simpler. If the dirstateguard constructor raises an exception, the finally-block won't do anything anyway, so this is functionally equivalent (and there is no except-block). Differential Revision: https://phab.mercurial-scm.org/D405

copies: add more details to the documentation of mergecopies() This documentation is very helpful for any developer to understand what copytracing is and what the function does. Since this is the main function of doing copytracing, I have also included bits about copytracing in it. This additions are picked from a doc by Stash@Fb. So thanks to him. Differential Revision: https://phab.mercurial-scm.org/D409