refs: Speed up packed lookups.
Currently ref lookups require loading the entire packed-refs file into
a hashmap in memory. For repos with large numbers of refs this can be
painfully slow.
This patch replaces the existing lookup code and instead mmap()'s the
packed-refs file and performs a binary search to locate the ref entry.
Git uses a similiar approach.
The old hash table codepath is still used for unsorted packed-refs files.
This patch also fixes a minor bug where the "peeled" trait is never
parsed correctly from the packed-refs header.
Ported t04_commit.c to Clar.
Created a copy of tests/resources/testrepo.git that is compatible
with the Clar sandboxing helpers.
Restructured commit test suites to use Clar sandbox helpers.
Now using typed data arrays rather than lots of macros to define test
cases.
