rerere: rerere_created_at() and has_resolution() abstraction

rerere: rerere_created_at() and has_resolution() abstraction

There were too many places in the code how an entry in the rerere database
looks like, and the garbage_collect() function that iterates over
subdirectories of the rr-cache directory was the worse offender.

Introduce two helper functions, rerere_created_at() and has_resolution(),
to abstract out the logic a bit better.

Incidentally this fixes a small memory leak in garbage_collect()
function.  The path list to collect the entries to be pruned were defined
to strdup the paths but the caller was feeding a path after doing an extra
copy.  Because the list does not have to be sorted by conflict signature
hash, we use path_list_append() instead of path_list_insert().

While we are at it, make a conflicted hunk comparision in handle_file() a
bit easier to read.

Signed-off-by: Junio C Hamano <gitster@pobox.com>