Make xstrndup common

This also improves the implementation to match how strndup is
specified (by GNU): if the length given is longer than the string,
only the string's length is allocated and copied, but the string need
not be null-terminated if it is at least as long as the given length.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

commit.c

@@ -720,14 +720,6 @@ static char *logmsg_reencode(const struct commit *commit,
 	return out;
 }
 
-static char *xstrndup(const char *text, int len)
-{
-	char *result = xmalloc(len + 1);
-	memcpy(result, text, len);
-	result[len] = '\0';
-	return result;
-}
-
 static void fill_person(struct interp *table, const char *msg, int len)
 {
 	int start, end, tz = 0;