prefixcmp(): fix-up mechanical conversion.

prefixcmp(): fix-up mechanical conversion.

Previous step converted use of strncmp() with literal string
mechanically even when the result is only used as a boolean:

    if (!strncmp("foo", arg, 3)) ==> if (!(-prefixcmp(arg, "foo")))

This step manually cleans them up to read:

    if (!prefixcmp(arg, "foo"))

Signed-off-by: Junio C Hamano <junkio@cox.net>