sparse-index: expand_to_path()

Some users of the index API have a specific path they are looking for,
but choose to use index_file_exists() to rely on the name-hash hashtable
instead of doing binary search with index_name_pos(). These users only
need to know a yes/no answer, not a position within the cache array.

When the index is sparse, the name-hash hash table does not contain the
full list of paths within sparse directories. It _does_ contain the
directory names for the sparse-directory entries.

Create a helper function, expand_to_path(), for intended use with the
name-hash hashtable functions. The integration with name-hash.c will
follow in a later change.

The solution here is to use ensure_full_index() when we determine that
the requested path is within a sparse directory entry. This will
populate the name-hash hashtable as the index is recomputed from
scratch.

There may be cases where the caller is trying to find an untracked path
that is not in the index but also is not within a sparse directory
entry. We want to minimize the overhead for these requests. If we used
index_name_pos() to find the insertion order of the path, then we could
determine from that position if a sparse-directory exists. (In fact,
just calling index_name_pos() in that case would lead to expanding the
index to a full index.) However, this takes O(log N) time where N is the
number of cache entries.

To keep the performance of this call based mostly on the input string,
use index_file_exists() to look for the ancestors of the path. Using the
heuristic that a sparse directory is likely to have a small number of
parent directories, we start from the bottom and build up. Use a string
buffer to allow mutating the path name to terminate after each slash for
each hashset test.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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