test-show: make it compatible with chg

test-show: make it compatible with chg

The show extension reads `commands.show.aliasprefix` config in its
`extsetup` and that causes issues with chg. This patch adds that config item
to chg confighash to solve the issue.

Test Plan:
`run-tests.py -l --chg test-show.t`

Differential Revision: https://phab.mercurial-scm.org/D1158

--- a/mercurial/chgserver.py	Tue Oct 17 10:51:43 2017 -0400
+++ b/mercurial/chgserver.py	Tue Oct 17 10:41:56 2017 -0700
@@ -73,6 +73,10 @@
     'extensions',
 ]
 
+_configsectionitems = [
+    ('commands', 'show.aliasprefix'), # show.py reads it in extsetup
+]
+
 # sensitive environment variables affecting confighash
 _envre = re.compile(r'''\A(?:
                     CHGHG
@@ -101,6 +105,8 @@
     sectionitems = []
     for section in _configsections:
         sectionitems.append(ui.configitems(section))
+    for section, item in _configsectionitems:
+        sectionitems.append(ui.config(section, item))
     sectionhash = _hashlist(sectionitems)
     envitems = [(k, v) for k, v in encoding.environ.iteritems()
                 if _envre.match(k)]