drop length argument of has_extension

drop length argument of has_extension

As Fredrik points out the current interface of has_extension() is
potentially confusing.  Its parameters include both a nul-terminated
string and a length-limited string.

This patch drops the length argument, requiring two nul-terminated
strings; all callsites are updated.  I checked that all of them indeed
provide nul-terminated strings.  Filenames need to be nul-terminated
anyway if they are to be passed to open() etc.  The performance penalty
of the additional strlen() is negligible compared to the system calls
which inevitably surround has_extension() calls.

Additionally, change has_extension() to use size_t inside instead of
int, as that is the exact type strlen() returns and memcmp() expects.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>