Add giffdiff functions

[estebanlm]

this allows me to make a diff compare using lib git mechanism. Is barebones but it may be used in the future to enhance some merge info.
Usage:

LGitLibrary uniqueInstance initializeLibGit2.

s1 := 'xx1
yy
zz'.
s2 := 'xx2
xy
zz'.

diff := LGitDiff basicNew.

opts := LGitDiffOptions defaults.

lines := OrderedCollection new.
lineCallback := LGitDiffLineCallback on: [ :delta :hunk :line :payload |
	lines add: { line prim_origin. line prim_new_lineno. line content }. 
	0 ].

diff 
	diff_buffers: s1 
	old_len: s1 size  
	old_as_path: 'old.st' 
	new_buffer: s2 
	new_len: s2 size 
	new_as_path: 'new.st' 
	options: opts 
	file_cb: ExternalAddress null 
	binary_cb: ExternalAddress null
	hunk_cb: ExternalAddress null
	line_cb: lineCallback 
	payload: ExternalAddress null. "a LGitReturnCodeEnum(#git_ok [0])"

lines