userdiff-cpp: tighten word regex

userdiff-cpp: tighten word regex

Generally, word regex can be written such that they match tokens
liberally and need not model the actual syntax because it can be assumed
that the regex will only be applied to syntactically correct text.

The regex for cpp (C/C++) is too liberal, though. It regards these
sequences as single tokens:

   1+2
   1.5-e+2+f

and the following amalgams as one token:

   .l      as in str.length
   .f      as in str.find
   .e      as in str.erase

Tighten the regex in the following way:

- Accept + and - only in one position in the exponent. + and - are no
  longer regarded as the sign of a number and are treated by the
  catcher-all that is not visible in the driver's regex.

- Accept a leading decimal point only when it is followed by a digit.

For readability, factor hex- and binary numbers into an own term.

As a drive-by, this fixes that floating point numbers such as 12E5
(with upper-case E) were split into two tokens.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>