git-arc: Trap on every mktemp

Trap:

• EXIT (0)
• HUP (1)
• INT (2)
• QUIT (3)
• TRAP (5)
• USR1 (10)
• TERM (15)

every time mktemp is called to reduce the chances of leaving stray files
or directories with possible sensitive data inside.

We avoid using a template with mktemp, as some operating systems may use
unpredictable base paths by default (macOS).

Suggested by: des
Reviewed by: emaste, 0mp, des (earlier), markj
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D47289