The commit time is already stored as a git_time_t, but we were
parsing is as a uint32_t. This just switches the parser to use
uint64_t which will handle dates further in the future (and adds
some tests of those future dates).
This moves the check for the "encoding" header into a loop which
is just scanning for non-required headers at the end of a commit
header. That loop will skip unrecognized lines (including header
continuation lines) until a terminating completely blank line is
found, and only then does it move to reading the commit message.
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.
