hgext: rename show.py to fbshow.py to prevent upstream collision

Summary:
Upstream recently introduced show as an extension. Because the file
name is the same and it lives in core, extensions.show now points to it.

It behaves differently. Rename our extension. In the next diff I'll update
facebook.rc to make extensions.show point to our version.

Test Plan: Ran tests

Reviewers: durham, #sourcecontrol

Subscribers: mjpieters

Differential Revision: https://phabricator.intern.facebook.com/D4845959

hgext3rd/fbshow.py

@@ -1,4 +1,4 @@
-# show.py
+# fbshow.py
 #
 # Copyright 2016 Facebook, Inc.
 #

tests/test-dialect-show.t

@@ -1,7 +1,7 @@
   $ cat >> $HGRCPATH << EOF
   > [extensions]
   > dialect = $TESTDIR/../hgext3rd/dialect.py
-  > show = $TESTDIR/../hgext3rd/show.py
+  > show = $TESTDIR/../hgext3rd/fbshow.py
   > EOF
 
   $ hg help -e show | head -n 1

tests/test-show.t

@@ -1,8 +1,8 @@
   $ extpath=`dirname $TESTDIR`
-  $ cp $extpath/hgext3rd/show.py $TESTTMP # use $TESTTMP substitution in message
+  $ cp $extpath/hgext3rd/fbshow.py $TESTTMP # use $TESTTMP substitution in message
   $ cat >> $HGRCPATH << EOF
   > [extensions]
-  > show=$TESTTMP/show.py
+  > show=$TESTTMP/fbshow.py
   > EOF
 
 We assume that log basically works (it has its own tests). This just covers uses