color: document the possibility to colourise tabs in diffs

This is a no-op change that simply suggests that tabs can be
colourised, but defaults to no colour effects for tabs. This
complements cset c343557a8442.

Like the similar effects for phases from f8e2aebbb24c, we set the
default effects to '' instead of None, so that this is a true noop
change. Otherwise, the diff.tab effect would override (i.e.
neutralise) the effect of the surrounding label.

hgext/color.py

@@ -11,7 +11,7 b' This extension modifies the status and r'
 to their output to reflect file status, the qseries command to add
 color to reflect patch status (applied, unapplied, missing), and to
 diff-related commands to highlight additions, removals, diff headers,
-and trailing whitespace.
+tabs, and trailing whitespace.
 
 Other effects in addition to color, like bold and underlined text, are
 also available. By default, the terminfo database is used to find the
@@ -61,6 +61,7 b' effects may be overridden from your conf'
   diff.deleted = red
   diff.inserted = green
   diff.changed = white
+  diff.tab =
   diff.trailingwhitespace = bold red_background
 
   # Blank so it inherits the style of the surrounding label
@@ -279,6 +280,7 b' except ImportError:'
            'diff.file_b': 'green bold',
            'diff.hunk': 'magenta',
            'diff.inserted': 'green',
+           'diff.tab': '',
            'diff.trailingwhitespace': 'bold red_background',
            'changeset.public' : '',
            'changeset.draft' : '',