$ git init
Initialized empty Git repository in /tmp/t/.git/
$ git rev-parse ffffffffffffffffffffffffffffffffffffffff^@
Segmentation fault (core dumped)
$ rpm -q git
git-2.14.3-3.fc27.x86_64

GDB shows this:

(gdb) break lookup_commit_reference
Breakpoint 1 at 0x555555609f00: lookup_commit_reference. (3 locations)
(gdb) r
Starting program: /usr/bin/git rev-parse ffffffffffffffffffffffffffffffffffffffff\^@
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Breakpoint 1, lookup_commit_reference (oid=oid@entry=0x7fffffffd550) at commit.c:34
34		return lookup_commit_reference_gently(oid, 0);
(gdb) finish
Run till exit from #0  lookup_commit_reference (oid=oid@entry=0x7fffffffd550) at commit.c:34
try_parent_shorthands (arg=0x7fffffffdd44 'f' <repeats 40 times>) at builtin/rev-parse.c:314
314			include_parents = 1;
Value returned is $1 = (struct commit *) 0x0
(gdb) c

(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
try_parent_shorthands (arg=0x7fffffffdd44 'f' <repeats 40 times>) at builtin/rev-parse.c:345
345		for (parents = commit->parents, parent_number = 1;
(gdb) l 336,+15
336		commit = lookup_commit_reference(&oid);
337		if (exclude_parent &&
338		    exclude_parent > commit_list_count(commit->parents)) {
339			*dotdot = '^';
340			return 0;
341		}
342	
343		if (include_rev)
344			show_rev(NORMAL, &oid, arg);
345		for (parents = commit->parents, parent_number = 1;
346		     parents;
347		     parents = parents->next, parent_number++) {
348			char *name = NULL;
349	
350			if (exclude_parent && parent_number != exclude_parent)
351				continue;

Looks like a null pointer check is missing.