templater: replace 'ctx._repo' with 'ctx.repo()'

largefiles: replace 'ctx._repo' with 'ctx.repo()'

obsolete: replace 'ctx._repo' with 'ctx.repo()'

fileset: replace 'ctx._repo' with 'ctx.repo()'

filectx: add a repo accessor This is similar to a07314472a80 in motivation. All contexts now have this method, so the rest of the 'ctx._repo' uses can be converted without worrying about what type of context it is.

run-tests: ignore ENOENT failures when removing old .err results When the same test runs in multiple threads and the previous run was a failure, the threads can race to delete the error output. This fixes that.

run-tests: stop storing start/stop times in a dict by test name This resolves the last breakage in run-tests that prevented me from running a single test many times in several threads in parallel. This will be useful for testing potential fixes to flaky tests.

run-tests: avoid running the same test instance concurrently There's a fair amount of mutable state stored on test case instances. That causes many weird failures if you try to do something like `run-tests.py -j16 --loop test-help.t`. The quick fix is this slightly weird test-reloading dance, which ensures that every time a test is executed it runs on a fresh instance of the TestCase subclass.

run-tests: add --runs-per-test flag This is useful when you're working with a flaky test and want to run it (for example) 500 times to see if it'll false-fail. This currently breaks if you use it with more than one thread, but I'm looking into that now.

convert: adjust progress bar for octopus merges (issue4169) For merges, we walk the files N-1 times, where N is the number of parents. This means that for an octopus merge with 3 parents and 2 changed files, we actually fetch 6 files. This corrects the progress output of the convert command when such commits are encountered.