gitweb: Teach "a=blob" action to be more lenient about blob/file mime type

gitweb: Teach "a=blob" action to be more lenient about blob/file mime type

Since 930cf7dd7cc6b87d173f182230763e1f1913d319 'blob' action knows the
file type; if the file type is not "text/*" or one of common network
image formats/mimetypes (gif, png, jpeg) then the action "blob"
defaulted to "blob_plain".  This caused the problem if mimetypes file
was not well suited for web, for example returning "application/x-sh"
for "*.sh" shell scripts, instead of "text/plain" (or other "text/*").

Now "blob" action defaults to "blob_plain" ('raw' view) only if file
is of type which is neither "text/*" nor "image/{gif,png,jpeg}"
AND it is binary file.  Otherwise it assumes that it can be displayed
either in <img> tag ("image/*" mimetype), or can be displayed line by
line (otherwise).

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