trim_last_path_component(): replace last_path_elm()

trim_last_path_component(): replace last_path_elm()

Rewrite last_path_elm() to take a strbuf parameter and to trim off the
last path name element in place rather than returning a pointer to the
beginning of the last path name element. This simplifies the function
a bit and makes it integrate better with its caller, which is now also
strbuf-based. Rename the function accordingly and a bit less tersely.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>