path: use size_t for dir_prefix length

path: use size_t for dir_prefix length

The strlen() function returns a size_t. Storing this in a standard
signed int is a bad practice that invites overflow vulnerabilities if
paths get absurdly long.

Switch the variable to size_t. This is safe to do because 'len' is
strictly used as an argument to strncmp() (which expects size_t) and
as a positive array index, involving no signed arithmetic that could
rely on negative values.

Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>