When doing copy detection, it is often necessary to include
UNMODIFIED records in the git_diff so they are available as source
records for GIT_DIFF_FIND_COPIES_FROM_UNMODIFIED. Yet in the final
diff, often you will not want to have these UNMODIFIED records.
This adds a flag which marks these UNMODIFIED records for deletion
from the diff list so they will be removed after the rename detect
phase is over.
When FIND_COPIES is used in combination with BREAK_REWRITES for
rename detection, there was a bug where the split MODIFIED delta
was only used as a target for RENAME records and not for COPIED
records. This fixes that, converting the split into a pair of
DELETED and COPIED deltas when that circumstance arises.
