tests: add test showing broken extension loading in case of share-safe

When we are using share-safe and loading source repository config, we are still
not loading the extensions which are enabled in the source repository.

This patch demonstrates the bug. Meanwhile it also shows how unreliable `hg
debugextensions` is.

Differential Revision: https://phab.mercurial-scm.org/D9046

tests/test-share-safe.t

@@ -81,6 +81,43 b' Check that config of the source reposito'
   $ hg showconfig ui.curses
   true
 
+Test that extensions of source repository are also loaded
+
+  $ hg debugextensions
+  share
+  $ hg extdiff -p echo
+  hg: unknown command 'extdiff'
+  'extdiff' is provided by the following extension:
+  
+      extdiff       command to allow external programs to compare revisions
+  
+  (use 'hg help extensions' for information on enabling extensions)
+  [255]
+
+  $ echo "[extensions]" >> ../source/.hg/hgrc
+  $ echo "extdiff=" >> ../source/.hg/hgrc
+
+  $ hg debugextensions -R ../source
+  extdiff
+  share
+  $ hg extdiff -R ../source -p echo
+
+BROKEN: the command below does not work but debugextensions says that extension
+is loaded
+  $ hg debugextensions
+  extdiff
+  share
+
+BROKEN: extdiff command should work here
+  $ hg extdiff -p echo
+  hg: unknown command 'extdiff'
+  'extdiff' is provided by the following extension:
+  
+      extdiff       command to allow external programs to compare revisions
+  
+  (use 'hg help extensions' for information on enabling extensions)
+  [255]
+
 However, local .hg/hgrc should override the config set by share source
 
   $ echo "[ui]" >> .hg/hgrc
@@ -92,6 +129,8 b' However, local .hg/hgrc should override '
   $ HGEDITOR=cat hg config --shared
   [ui]
   curses=true
+  [extensions]
+  extdiff=
 
   $ HGEDITOR=cat hg config --local
   [ui]