debugcolor: fix crash by empty styles (issue5856)

mercurial/debugcommands.py

@@ -462,6 +462,8 b' def _debugdisplaycolor(ui):'
 
 def _debugdisplaystyle(ui):
     ui.write(_('available style:\n'))
+    if not ui._styles:
+        return
     width = max(len(s) for s in ui._styles)
     for label, effects in sorted(ui._styles.items()):
         ui.write('%s' % label, label=label)

tests/test-debugcommands.t

@@ -333,6 +333,19 b' Test cache warming command'
   .hg/cache/rbc-names-v1
   .hg/cache/branch2-served
 
+Test debugcolor
+
+#if no-windows
+  $ hg debugcolor --style --color always | egrep 'mode|style|log\.'
+  color mode: ansi
+  available style:
+  \x1b[0;33mlog.changeset\x1b[0m:                      \x1b[0;33myellow\x1b[0m (esc)
+#endif
+
+  $ hg debugcolor --style --color never
+  color mode: None
+  available style:
+
   $ cd ..
 
 Test internal debugstacktrace command