builtin/rebase: release git_format_patch_opt too

options.git_format_patch_opt can be populated during cmd_rebase's setup,
and will therefore leak on return. Although we could just UNLEAK all of
options, we choose to strbuf_release() the individual member, which matches
the existing pattern (where we're freeing invidual members of options).

Leak found when running t0021:

Direct leak of 24 byte(s) in 1 object(s) allocated from:
    #0 0x49ab79 in realloc ../projects/compiler-rt/lib/asan/asan_malloc_linux.cpp:164:3
    #1 0x9ac296 in xrealloc wrapper.c:126:8
    #2 0x93b13d in strbuf_grow strbuf.c:98:2
    #3 0x93bd3a in strbuf_add strbuf.c:295:2
    #4 0x60ae92 in strbuf_addstr strbuf.h:304:2
    #5 0x605f17 in cmd_rebase builtin/rebase.c:1759:3
    #6 0x4cd91d in run_builtin git.c:467:11
    #7 0x4cb5f3 in handle_builtin git.c:719:3
    #8 0x4ccf47 in run_argv git.c:808:4
    #9 0x4caf49 in cmd_main git.c:939:19
    #10 0x69dbfe in main common-main.c:52:11
    #11 0x7f66dae91349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: 24 byte(s) leaked in 1 allocation(s).

Signed-off-by: Andrzej Hunt <ajrhunt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -2119,6 +2119,7 @@ cleanup:
 	free(options.head_name);
 	free(options.gpg_sign_opt);
 	free(options.cmd);
+	strbuf_release(&options.git_format_patch_opt);
 	free(squash_onto_name);
 	return ret;
 }