gitweb: Handle non UTF-8 text better

gitweb assumes that everything is in UTF-8. If a text contains invalid
UTF-8 character sequences, the text must be in a different encoding.

This commit introduces $fallback_encoding which would be used as input
encoding if gitweb encounters text with is not valid UTF-8.

Add basic test for this in t/t9500-gitweb-standalone-no-errors.sh

Signed-off-by: Martin Koegler <redacted>
Signed-off-by: Jakub Narebski <redacted>
Tested-by: Alexandre Julliard <redacted>
Tested-by: Ismail Dönmez <redacted>
Signed-off-by: Junio C Hamano <redacted>