Cleanup of unused symcache variable inside diff-lib.c

Cleanup of unused symcache variable inside diff-lib.c

Commit c40641b77b0274186fd1b327d5dc3246f814aaaf, 'Optimize
symlink/directory detection' by Linus Torvalds, removed the 'char
*symcache' parameter to the has_symlink_leading_path() function.  This
made all variables currently named 'symcache' inside diff-lib.c
unnecessary.

This also let us throw away the 'struct oneway_unpack_data', and
instead directly use the 'struct rev_info *revs' member, which
was the only member left after removal of the 'symcache[] array'
member.  The 'struct oneway_unpack_data' was introduced by the
following commit:

  948dd346  "diff-files: careful when inspecting work tree items"

Impact: cleanup
        PATH_MAX bytes less memory stack usage in some cases

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>