Fix bug making split deltas a COPIED targets

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.
Add GIT_DIFF_FIND_DELETE_UNMODIFIED flag

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.
GIT_DIFF_FIND_REMOVE_UNMODIFIED sounds better
Merge pull request #1979 from libgit2/rb/diff-find-delete-unmod

Add GIT_DIFF_FIND_REMOVE_UNMODIFIED flag and fix copy detection bug