builtin/checkout: clear pending objects after diffing

builtin/checkout: clear pending objects after diffing

add_pending_object() populates rev.pending, we need to take care of
clearing it once we're done.

This code is run close to the end of a checkout, therefore this leak
seems like it would have very little impact. See also LSAN output
from t0020 below:

Direct leak of 2048 byte(s) in 1 object(s) allocated from:
    #0 0x49ab79 in realloc ../projects/compiler-rt/lib/asan/asan_malloc_linux.cpp:164:3
    #1 0x9acc46 in xrealloc wrapper.c:126:8
    #2 0x83e3a3 in add_object_array_with_path object.c:337:3
    #3 0x8f672a in add_pending_object_with_path revision.c:329:2
    #4 0x8eaeab in add_pending_object_with_mode revision.c:336:2
    #5 0x8eae9d in add_pending_object revision.c:342:2
    #6 0x5154a0 in show_local_changes builtin/checkout.c:602:2
    #7 0x513b00 in merge_working_tree builtin/checkout.c:979:3
    #8 0x512cb3 in switch_branches builtin/checkout.c:1242:9
    #9 0x50f8de in checkout_branch builtin/checkout.c:1646:9
    #10 0x50ba12 in checkout_main builtin/checkout.c:2003:9
    #11 0x5086c0 in cmd_checkout builtin/checkout.c:2055:8
    #12 0x4cd91d in run_builtin git.c:467:11
    #13 0x4cb5f3 in handle_builtin git.c:719:3
    #14 0x4ccf47 in run_argv git.c:808:4
    #15 0x4caf49 in cmd_main git.c:939:19
    #16 0x69e43e in main common-main.c:52:11
    #17 0x7f5dd1d50349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: 2048 byte(s) leaked in 1 allocation(s).
Signed-off-by: Andrzej Hunt <ajrhunt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>