rerere: remove duplicated functions

rerere: remove duplicated functions

Both rerere.c and builtin-rerere.c define the static functions
rr_path() and has_resolution() the exact same way.  To eliminate this
code duplication this patch turns the functions in rerere.c
non-static, and makes builtin-rerere.c use them.  Also, since this
puts these two functions into the global namespace, rename them to
rerere_path() and has_rerere_resolution(), respectively, and rename
their "name" parameter to "hex", because it better reflects what that
parameter actually is.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>