hashmap: allow memihash computation to be continued

hashmap: allow memihash computation to be continued

Add variant of memihash() to allow the hash computation to
be continued.  There are times when we compute the hash on
a full path and then the hash on just the path to the parent
directory.  This can be expensive on large repositories.

With this, we can hash the parent directory first. And then
continue the computation to include the "/filename".

Signed-off-by: Jeff Hostetler <redacted>
Signed-off-by: Junio C Hamano <redacted>