wrapper: implement xfopen()

wrapper: implement xfopen()

A common usage pattern of fopen() is to check if it succeeded, and die()
if it failed:

	FILE *fp = fopen(path, "w");
	if (!fp)
		die_errno(_("could not open '%s' for writing"), path);

Implement a wrapper function xfopen() for the above, so that we can save
a few lines of code and make the die() messages consistent.

Helped-by: Jeff King <redacted>
Helped-by: Johannes Schindelin <redacted>
Helped-by: Junio C Hamano <redacted>
Signed-off-by: Paul Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>