CodingGuidelines: allow grep -E

Despite forbidden by CodingGuidelines, our usage of 'grep -E' has been
increased over the years, and noone has come and complained.

Let's lift the restriction.

Signed-off-by: Đoàn Trần Công Danh <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: sgn

Documentation/CodingGuidelines

@@ -162,8 +162,6 @@ For shell scripts specifically (not exhaustive):
 
    - We do not use \{m,n\};
 
-   - We do not use -E;
-
    - We do not use ? or + (which are \{0,1\} and \{1,\}
      respectively in BRE) but that goes without saying as these
      are ERE elements not BRE (note that \? and \+ are not even part