debuginstall: add Python information to debuginstall output (issue4128)

This change adds to the output of "hg debuginstall" information about the
Python being used by Mercurial.  It adds both the path to the Python
executable (i.e. the value of sys.executable) and the version of Python
(specifically the major, minor, and micro versions).

Below is an example of what the output looks like after this change.
The marked lines are the new output lines:

    $ hg debuginstall
    checking encoding (UTF-8)...
 -->showing Python executable (/Users/chris/.virtualenvs/default/bin/python)
 -->showing Python version (2.7.6)
    checking Python lib (/Users/chris/.virtualenvs/default/lib/python2.7)...
    checking installed modules (/Users/chris/mercurial)...
    checking templates (/Users/chris/mercurial/templates)...
    checking commit editor...
    checking username...
    no problems detected

Note that we use the word "showing" without an ellipsis for the new lines
because, unlike the other lines (except for "Python lib" which will be
adjusted in a subsequent commit), no check follows the display of this
information.

mercurial/commands.py

@@ -9,6 +9,7 b' from node import hex, bin, nullid, nullr'
 from lock import release
 from i18n import _
 import os, re, difflib, time, tempfile, errno
+import sys
 import hg, scmutil, util, revlog, copies, error, bookmarks
 import patch, help, encoding, templatekw, discovery
 import archival, changegroup, cmdutil, hbisect
@@ -2159,7 +2160,10 b' def debuginstall(ui):'
         ui.write(_(" (check that your locale is properly set)\n"))
         problems += 1
 
-    # Python lib
+    # Python
+    ui.status(_("showing Python executable (%s)\n") % sys.executable)
+    ui.status(_("showing Python version (%s)\n")
+              % ("%s.%s.%s" % sys.version_info[:3]))
     ui.status(_("checking Python lib (%s)...\n")
               % os.path.dirname(os.__file__))
 

tests/test-install.t

@@ -1,6 +1,8 b''
 hg debuginstall
   $ hg debuginstall
   checking encoding (ascii)...
+  showing Python executable (*) (glob)
+  showing Python version (2.*) (glob)
   checking Python lib (*lib*)... (glob)
   checking installed modules (*mercurial)... (glob)
   checking templates (*mercurial?templates)... (glob)
@@ -11,6 +13,8 b' hg debuginstall'
 hg debuginstall with no username
   $ HGUSER= hg debuginstall
   checking encoding (ascii)...
+  showing Python executable (*) (glob)
+  showing Python version (2.*) (glob)
   checking Python lib (*lib*)... (glob)
   checking installed modules (*mercurial)... (glob)
   checking templates (*mercurial?templates)... (glob)