Use starts_with() for C strings instead of memcmp()

Use starts_with() for C strings instead of memcmp()

Convert three cases of checking for a constant prefix using memcmp() to
starts_with().  This way there is no need for magic string length
constants and we avoid running over the end of the string should it be
shorter than the prefix.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>