Invoke less pager command in a subshell

This is done so that unsetting environment variable via `LESS=` works in
all shells.
For example, unsetting variable in `fish` shell, the command to use is

  env LESS= less ...

To avoid handling all the cases we're executing the whole command in
plain `sh`.

Author: Bruno Sutic

CHANGELOG.md

@@ -4,6 +4,8 @@
 - do not colorize `tree` output because a pager application might not handle it
   well
 - do not suggest using `more` program in the readme
+- invoke `less` pager command in a subshell so that unsetting environment
+  variable via `LESS=` works in any shell
 
 ### v0.7.0, Sep 05, 2014
 - bugfix: invalid params for 'save_sidebar_width' script

scripts/variables.sh

@@ -12,7 +12,7 @@ TREE_FOCUS_OPTION="@sidebar-tree-focus"
 TREE_COMMAND="tree"
 TREE_COMMAND_OPTION="@sidebar-tree-command"
 
-TREE_PAGER="LESS= less --dumb --chop-long-lines --tilde --IGNORE-CASE --RAW-CONTROL-CHARS"
+TREE_PAGER='sh -c "LESS= less --dumb --chop-long-lines --tilde --IGNORE-CASE --RAW-CONTROL-CHARS"'
 TREE_PAGER_OPTION="@sidebar-tree-pager"
 
 TREE_POSITION="left"