Allow to set the minimum required match length.

The minimum required match length plays an important role in the
current algorithm. This algorithm requires that the current match
has at least 8 mismatches over a region where we find a new match,
therefore imposing a minimum match length of at least 8 bytes.
This patch allows to increase this minium value to avoid using small
matches that only match a very small region. A larger value can
improve the patch size, depending on the data, but a too large value
will cause the algorithm to miss matches that were otherwise benefical.

Bug: 73107113
Test: Added unittests. Ran it with different minlen values.

Change-Id: Iac594fc1cd7ecd5cfdc676bdb6ebe7c626de6dcd