ui-blob.c: Use a context structure in walk_tree()

Do not misuse global variables to save the context. Instead, use the
context pointer which was designed to share information between a
read_tree_fn and the caller.

This also prevents from potential misuse of the global pointers
match_path and matched_sha1 after the referenced values have been
overwritten on the stack.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>