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.

test-convert-git: show insane progress bar with octopus merge (issue4169) This is done as a separate change from the fix so that it's obvious what is corrected by the bugfix.

manifest: have context use self.hasdir() A couple places in context currently use "x in self._dirs" to check for the existence of the directory, but this requires that all directories be loaded into a dict. Calling hasdir() instead puts the work on the the manifest to check for the existence of a directory in the most efficient manner.

manifest: add hasdir() to context This is a convenience method that calls to its manifest's hasdir(). There are parts of context that check to see if a directory exists, and this method will let implementations of manifest provide an optimal way to find a particular directory.

manifest: add manifestdict.hasdir() method Allows for alternative implementations of manifestdict to decide if a directory exists in whatever way is most optimal.

manifest: have context's dirs() call its manifest's dirs() This lets the context's dirs() method be agnostic towards any alternate manifest implementations.

manifest: add dirs() to manifestdict Manifests should have a method of accessing its own dirs, not just the context that references the manifest. This makes it easier for other optimized versions of manifests to compute their own dirs in the most efficient way.

formatter: convert None to json null It will be used by "annotate" command to represent the workingctx revision.