Because utils_strv_find_lcs() didn't consisider path component boundaries
it have have found substrings that are longest in itself but not so ideal
when utils_strv_shorten_file_list() applied path boundaries.
utils_strv_find_lcs() now can optionally restrict the substring between
delimiters (i.e. dir separators). In that mode it will find the longest
substring that is also sorrounded by the delimiters (there may be more
delimiters inside the string).
The unit test that demonstrated the deficient is fixed since
now the expected substitution takes place.
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