Histogram diff algorithm specifies MAX_CNT and MAX_PTR as upper limits
for cnt and ptr (both unsigned int's). This does not work, because
MAX_CNT / MAX_PTR are set to be UINT_MAX. Less-than comparisons against
them will succeed by default, meaning that the XDL_MIN check is a
logical no-op and fails to protect against `rec->cnt + 1` overflowing.
Additionally, xdiff in general is not safe to use above the 32-bit
integer limit, and Git protects against that with a hard cap of ~1 GB
(MAX_XDIFF_SIZE), meaning the histogram algorithm should never see cnt /
ptr reach these limits to begin with.

Tracing the lineage of the histogram algorithm, this appears to be a
porting error when transcribing from HistogramDiffIndex.java from
Eclipse jgit. In the original implementation, MAX_CNT and MAX_PTR are
set to much lower limits due to details specific to that implementation
(bit fields), which aren't relevant to xdiff's implementation.

To fix this, remove both the checks and MAX_CNT / MAX_PTR, as they don't
serve a purpose.

Signed-off-by: Yee Cheng Chin <[email protected]>