debuginstall: do not pass exception object to formatter (issue5676)

debuginstall: use codecs.lookup() to detect invalid encoding encoding.fromlocal() never tries to decode an ascii string since 853574db5b12, and there's no universal non-ascii string which can be decoded as any valid character set.

extensions: fix wrapcommand/function of class instance 5361771f9714 changed _updatewrapper() to copy the __name__ attribute, but not all callable objects has __name__. Spotted by loading mq with extdiff.

changegroup: avoid creating empty changegroup part Previously this check happened in the changegroup code itself. Since its refactor, this logic needs to move out to callers that care about it, such as this one. Otherwise we get empty bundle devel-warnings in certain extensions. Differential Revision: https://phab.mercurial-scm.org/D690

tests: split test-revset.t in half This test has gotten so large that running it can exceed the normal timeout on systems under load (like if we're running all the tests in parallel). This patch splits the test cleanly in half. Differential Revision: https://phab.mercurial-scm.org/D694

merge: flush any deferred writes just before recordupdates() ``recordupdates`` calls into the dirstate which requires the files to be there, so this is the last possible moment we can flush anything. Differential Revision: https://phab.mercurial-scm.org/D673