debugcommands: add debugserve command `hg serve --stdio` requires the exact command argument form `hg -R <path> serve --stdio` for security reasons. An upcoming commit will need to start an SSH protocol server process with custom settings. This commit creates a `hg debugserve` command for starting servers with custom options. There are no security restrictions and we can add options here that aren't appropriate for built-in commands. We currently only support starting an SSH protocol server using the process's stdio file descriptors. The server supports logging its I/O activity to a file descriptor number passed as a command argument. Differential Revision: https://phab.mercurial-scm.org/D2464

--- a/tests/test-completion.t	Sun Feb 25 11:16:09 2018 -0800
+++ b/tests/test-completion.t	Tue Feb 27 15:47:44 2018 -0800
@@ -111,6 +111,7 @@
   debugrename
   debugrevlog
   debugrevspec
+  debugserve
   debugsetparents
   debugssl
   debugsub
@@ -291,6 +292,7 @@
   debugrename: rev
   debugrevlog: changelog, manifest, dir, dump
   debugrevspec: optimize, show-revs, show-set, show-stage, no-optimized, verify-optimized
+  debugserve: sshstdio, logiofd, logiofile
   debugsetparents: 
   debugssl: 
   debugsub: rev