posixworker: avoid creating workers that end up getting no work

If `workers` (the detected or configured number of CPUs) is greater
than the number of work items, then some of the workers end up getting
0 work items. Let's not create such workers.

Differential Revision: https://phab.mercurial-scm.org/D8927

mercurial/worker.py

@@ -211,7 +211,7 b' def _posixworker(ui, func, staticargs, a'
     parentpid = os.getpid()
     pipes = []
     retval = {}
-    for pargs in partition(args, workers):
+    for pargs in partition(args, min(workers, len(args))):
         # Every worker gets its own pipe to send results on, so we don't have to
         # implement atomic writes larger than PIPE_BUF. Each forked process has
         # its own pipe's descriptors in the local variables, and the parent