add '--numprocesses' as long option for '-n'

This is more verbose in scripts that run pytest-xdist. It can also help
in some ci-helper scripts like to auto detect if  pytest-xdist should be
installed.

changelog/168.feature

@@ -0,0 +1 @@
+Add long option `--numprocesses` as alternative for `-n`.

testing/test_plugin.py

@@ -20,6 +20,10 @@ def test_dist_options(testdir):
     check_options(config)
     assert config.option.dist == "load"
     assert config.option.tx == ['popen'] * 2
+    config = testdir.parseconfigure("--numprocesses 2")
+    check_options(config)
+    assert config.option.dist == "load"
+    assert config.option.tx == ['popen'] * 2
     config = testdir.parseconfigure("-d")
     check_options(config)
     assert config.option.dist == "load"

xdist/plugin.py

@@ -20,7 +20,7 @@ def parse_numprocesses(s):
 def pytest_addoption(parser):
     group = parser.getgroup("xdist", "distributed and subprocess testing")
     group._addoption(
-        '-n', dest="numprocesses", metavar="numprocesses",
+        '-n', '--numprocesses', dest="numprocesses", metavar="numprocesses",
         action="store",
         type=parse_numprocesses,
         help="shortcut for '--dist=load --tx=NUM*popen', "