A delta in the subversion delta (svndiff0) format consists of the
magic bytes SVN\0 followed by a sequence of windows of a certain well
specified format (starting with five integers).

Add an svndiff0_apply function and test-svn-fe -d commandline tool to
parse such a delta in the special case of not including any windows.

Later patches will add features to turn this into a fully functional
delta applier for svn-fe to use to parse the streams produced by
"svnrdump dump" and "svnadmin dump --deltas".

The content of symlinks starts with the word "link " in Subversion's
worldview, so we need to be able to prepend that text to input for the
sake of delta application.  So initialization of the input state of
the delta preimage is left to the calling program, giving callers a
chance to seed the buffer with text of their choice.

Improved-by: Ramkumar Ramachandra <artagnon@gmail.com>
Improved-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>