test(vcs): Add a VCS push hook to run tests

Personally, I can't stand working without a VCS hook anymore.  It saves me so much time
in context switches from revisiting and revising PRs when I see that the CI run is red.
Much better to fail before I push so I can revise while the relevant changes are fresh
in my head.

In order to start using this, one has to run `$ make build` first.  Should I add that
and make other documentation changes in the wiki related to my changes in this PR?

.pre-commit-config.yaml

@@ -0,0 +1,8 @@
+repos:
+  - repo: local
+    hooks:
+    - id: test
+      name: test
+      entry: "make test"
+      language: system
+      pass_filenames: false

Makefile

@@ -13,6 +13,8 @@ MAKEFLAGS += --warn-undefined-variables
 MAKEFLAGS += --no-builtin-rules
 
 # Local target variables
+VCS_HOOK_SAMPLES=$(wildcard .git/hooks/*.sample)
+VCS_HOOKS=$(VCS_HOOK_SAMPLES:%.sample=%)
 PYTHON=python
 export PYTHON
 PYFLAKES=flake8
@@ -27,13 +29,22 @@ APPNAME=tahoe-lafs
 default:
 	@echo "no default target"
 
+.PHONY: build
+## Set up and build for local development
+# To also run checks on every commit:
+# $ make .git/hooks/pre-commit
+build: .tox .git/hooks/pre-push
+
 .PHONY: test
 ## Run all tests and code reports
-test: .tox
-	tox --develop -p auto
+test: build
+# Run codechecks first since it takes the least time to report issues early.
+	tox --develop -e codechecks
+# Run all the test environments in parallel to reduce run-time
+	tox --develop -p auto -e 'py27,py36,pypy27'
 .PHONY: test-py3-all
 ## Run all tests under Python 3
-test-py3-all: .tox
+test-py3-all: build
 	tox --develop -e py36 allmydata
 
 # This is necessary only if you want to automatically produce a new
@@ -233,3 +244,6 @@ src/allmydata/_version.py:
 
 .tox: tox.ini setup.py
 	tox --notest -p all
+
+$(VCS_HOOKS): .tox .pre-commit-config.yaml
+	"./$(<)/py36/bin/pre-commit" install --hook-type $(@:.git/hooks/%=%)

tox.ini

@@ -37,6 +37,8 @@ deps =
      # regressions in new releases of this package that cause us the kind of
      # suffering we're trying to avoid with the above pins.
      certifi
+     # VCS hooks support
+     py36,!coverage: pre-commit
 
 # We add usedevelop=False because testing against a true installation gives
 # more useful results.
@@ -86,6 +88,13 @@ basepython = python2.7
 passenv = HOME
 whitelist_externals =
          /bin/mv
+setenv =
+         # Workaround an error when towncrier is run under the VCS hook,
+         # https://stackoverflow.com/a/4027726/624787:
+         #   File "/home/rpatterson/src/work/sfu/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/towncrier/check.py", line 44, in __main
+         #     .decode(getattr(sys.stdout, "encoding", "utf8"))
+         # `TypeError: decode() argument 1 must be string, not None`
+         PYTHONIOENCODING=utf_8
 commands =
          flake8 src static misc setup.py
          python misc/coding_tools/check-umids.py src