git and hg diff views did not show empty context lines, a regression

from the commit(s) from issue 633. The diff parser assumed a properly
formatted diff that denotes empty context lines with a single space in
the first column. This single space however was missing, because the
hg and git backends got the diff through PHP's exec() function and
this returns already line-splitted output, but - and this is the actual
problem - removes trailing whitespace at the end of each line, essentially
making " \n" only "\n". When splitting this string now again with
PREG_SPLIT_NO_EMPTY the empty line was completely lost in the diff output.

To make it clear that an empty line does not mark a context line now, but
should stop the diff parsing, the Diff parser now also defaults to 'false'
as line type.

This commit fixes issue 688.

NEWS.mdtext

@@ -14,6 +14,7 @@
 - Timeline only displays filter options for items a user has actually access to (issue 655)
 - The log, tags and branches parsers for Mercurial are more robust now (issue 663)
 - Fix SSH public key parsing issues and improve the check for existing, uploaded keys (issue 679)
+- Diff views now show empty context lines for git and hg again (issue 688)
 - Let the SVN command line client not store the login credentials we give him as arguments
 
 ## Documentation