Replaced all cl_assert(!strcmp()) or semantically equivalent forms
by cl_assert_equal_s().

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.

Without this change, any failed assertion in the second (or a later) test
inside a test suite has a chance of double deleting memory, resulting in
a heap corruption. See #1096 for details.

This leaves alone the test cases where we "just" use cl_git_sandbox_init()
and cl_git_sandbox_cleanup(). These methods already take good care to not
double delete a repository.

Fixes #1096

Signed-off-by: schu <schu-github@schulog.org>

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.

git_commit_create is supposed to update the given reference
"update_ref", but segfaulted in case of a yet to be born
reference. Fix it.

Signed-off-by: schu <schu-github@schulog.org>