run-tests: mechanism to report exceptions during test execution Sometimes when running tests you introduce a ton of exceptions. The most extreme example of this is running Mercurial with Python 3, which currently spews thousands of exceptions when running the test harness. This commit adds an opt-in feature to run-tests.py to aggregate exceptions encountered by `hg` when running tests. When --exceptions is used, the test harness enables the "logexceptions" extension in the test environment. This extension wraps the Mercurial function to handle exceptions and writes information about the exception to a random filename in a directory defined by the test harness via an environment variable. At the end of the test harness, these files are parsed, aggregated, and a list of all unique Mercurial frames triggering exceptions is printed in order of frequency. This feature is intended to aid Python 3 development. I've only really tested it on Python 3. There is no shortage of improvements that could be made. e.g. we could write a separate file containing the exception report - maybe even an HTML report. We also don't capture which tests demonstrate the exceptions, so there's no turnkey way to test whether a code change made an exception disappear. Perfect is the enemy of good. I think the current patch is useful enough to land. Whoever uses it can send patches to imprve its usefulness. Differential Revision: https://phab.mercurial-scm.org/D1477

run-tests: make --extra-config-opt work with Python 3 And add test coverage to ensure it works. Differential Revision: https://phab.mercurial-scm.org/D1476

run-tests: organize options into argument groups And sort arguments so help output is more legible. There are probably a ton of ways to group things. I tried to picture the test harness as a pipeline and attempted to draw boundaries around stages in that pipeline to create the groupings. Differential Revision: https://phab.mercurial-scm.org/D1475

run-tests: convert to argparse optparse has been deprecated since Python 3.2. Best to get on the new boat before the old one sinks. It looks like argparse formats its usage string differently than optparse. Meh. Differential Revision: https://phab.mercurial-scm.org/D1474

merge with stable

server: introduce a 'experimental.single-head-per-branch' option When the option is set, the repository will reject any transaction adding multiple heads to the same named branch. For now we reject all scenario with multiple heads. One could imagine handling closed branches differently. We prefer to keep things simple for now. The feature might get extended later. Branch closing is not the best experience Mercurial has to offer anyway.

scmutil: extra utility to display a reasonable amount of nodes Push have some logic to display a reasonable amount nodes. We extract it to an utility function to make it reusable.