mingw: simplify PATH handling

mingw: simplify PATH handling

On Windows the environment variable PATH contains a semicolon-separated
list of directories to search for, in order, when looking for the
location of a binary to run.  get_path_split() parses it and returns an
array of string copies, which is iterated by path_lookup(), which in
turn passes each entry to lookup_prog().

Change lookup_prog() to take the directory name as a length-limited
string instead of as a NUL-terminated one and parse PATH directly in
path_lookup().  This avoids memory allocations, simplifying the code.

Helped-by: Johannes Sixt <redacted>
Signed-off-by: Rene Scharfe <redacted>
Acked-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>