All the operations on the 'git_index_entry' array and the
'git_tree_entry' array have been refactored into common code in the
src/vector.c file.

The new vector methods support:
	- insertion:	O(1) (avg)
	- deletion:		O(n)
	- searching:	O(logn)
	- sorting:		O(logn)
	- r. access:	O(1)

Signed-off-by: Vicent Marti <tanoku@gmail.com>