color: restore _style global after debugcolor ran

Before this change, running 'debugcolor' would destroy all color style for the
rest of the process life. We now properly backup and restore the variable
content. Using a global variable is sketchy in general and could probably be
removed. However, this is a quest for another adventure.

hgext/color.py

@@ -540,6 +540,8 b' def extsetup(ui):'
 def debugcolor(ui, repo, **opts):
     """show available colors and effects"""
     global _styles
+    oldstyle = _styles
+    try:
     _styles = {}
     for effect in _effects.keys():
         _styles[effect] = effect
@@ -553,6 +555,8 b' def debugcolor(ui, repo, **opts):'
     ui.write(_('available colors:\n'))
     for colorname, label in _styles.items():
         ui.write(('%s\n') % colorname, label=label)
+    finally:
+        _styles = oldstyle
 
 if os.name != 'nt':
     w32effects = None