gitweb: Fix fallback mode of to_utf8 subroutine

gitweb: Fix fallback mode of to_utf8 subroutine

e5d3de5 (gitweb: use Perl built-in utf8 function for UTF-8 decoding.,
2007-12-04) was meant to make gitweb faster by using Perl's internals
(see subsection "Messing with Perl's Internals" in Encode(3pm) manpage)

Simple benchmark confirms that (old = 00f429a, new = this version):

        old  new
  old    -- -65%
  new  189%   --

Unfortunately it made fallback mode of to_utf8 do not work...  except
for default value 'latin1' of $fallback_encoding ('latin1' is Perl
native encoding), which is why it was not noticed for such long time.

utf8::valid(STRING) is an internal function that tests whether STRING
is in a _consistent state_ regarding UTF-8.  It returns true is
well-formed UTF-8 and has the UTF-8 flag on _*or*_ if string is held
as bytes (both these states are 'consistent').  For gitweb the second
option was true, as output from git commands is opened without ':utf8'
layer.

What made it work at all for STRING in 'latin1' encoding is the fact
that utf8:decode(STRING) turns on UTF-8 flag only if source string is
valid UTF-8 and contains multi-byte UTF-8 characters... and that if
string doesn't have UTF-8 flag set it is treated as in native Perl
encoding, i.e.  'latin1' / 'iso-8859-1' (unless native encoding it is
EBCDIC ;-)).  It was ':utf8' layer that actually converted 'latin1'
(no UTF-8 flag == native == 'latin1) to 'utf8'.

Let's make use of the fact that utf8:decode(STRING) returns false if
STRING is invalid as UTF-8 to check whether to enable fallback mode.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>