path: factor out skip_slashes() in normalize_path_copy_len()

path: factor out skip_slashes() in normalize_path_copy_len()

Extract skip_slashes() to avoid repeating the same is_dir_sep()
loop in multiple places inside normalize_path_copy_len().

Keep the dot-component handling inline to preserve the original
control flow and readability, as suggested in review.

No functional changes. Behavior verified with t0060-path-utils.sh.

Signed-off-by: Pushkar Singh <pushkarkumarsingh1970@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>