Fix detection of binary files

Images (at least PNG) were not detected as binary files.

Removed the sed call from getDiffFilesFromBranch(), and
replaced it with a PHP explode on '\t' instead of ' '.
getShowFile() now works for binary files.

Added a execReturnRaw() method, to execute a shell command
and return its output, without modifying / breaking it.

It uses passthru(), which doesn't modify the output, as opposite
to exec(), which ignores trailling whitespaces -- which means
binary files' content gets broken when using 'git show' with
exec().

Updated getShowFile() to use the new passthru()-base execReturnRaw()
method, instead of the exec()-based one.