pycompat: define operating system constants
As suggested by Ryan in D1019, it's cleaner if we use defined constants
instead of `osname == 'nt'` everywhere.
Differential Revision: https://phab.mercurial-scm.org/D1033
--- a/mercurial/pycompat.py Wed Oct 11 21:24:32 2017 -0700
+++ b/mercurial/pycompat.py Thu Oct 12 19:20:04 2017 -0700
@@ -316,3 +316,7 @@
rawinput = raw_input
isjython = sysplatform.startswith('java')
+
+isdarwin = sysplatform == 'darwin'
+isposix = osname == 'posix'
+iswindows = osname == 'nt'