Allow negative %d counts in pathmap pattern replacements

[afurm]

String#pathmap documents that %d takes a numeric prefix (including negative ones, e.g. %-2d) and that %d also accepts a {pattern,replacement} spec. Combining the two only works for positive counts:

"this/is/a/really/long/path/ok.rb".pathmap("%{/really/,/}5d")  # => "this/is/a/long"
"this/is/a/really/long/path/ok.rb".pathmap("%{path,dir}-2d")
# => ArgumentError: Unknown pathmap specifier %{path,dir}-2d in '%{path,dir}-2d'

The scanner already accepts %{...}-2d as one fragment (-?\d* in the scan regexp), but the branch that applies the replacement only matched \d*[dpfnxX], so the minus sign made it fall through to the "unknown specifier" error.

The replacement branch now matches -?\d+d or a bare operator, so %{path,dir}-2d returns "long/dir". A count is still only accepted together with d, so something like %{a,x}-2f keeps raising ArgumentError instead of being silently mangled.

Tests: added cases to test/test_rake_path_map.rb; the -2d one fails without the change. bundle exec rake test passes (607 tests) and RuboCop is clean on the changed files.