string_list API users: use string_list_init_{no,}dup

string_list API users: use string_list_init_{no,}dup

Follow-up on the introduction of string_list_init_nodup() and
string_list_init_dup() in the series merged in bd4232fac33 (Merge
branch 'ab/struct-init', 2021-07-16) and convert code that implicitly
relied on xcalloc() being equivalent to the initializer to use
xmalloc() and string_list_init_{no,}dup() instead.

In the case of get_unmerged() in merge-recursive.c we used the
combination of xcalloc() and assigning "1" to "strdup_strings" to get
what we'd get via string_list_init_dup(), let's use that instead.

Adjacent code in cmd_format_patch() will be changed in a subsequent
commit, since we're changing that let's change the other in-tree
patterns that do the same. Let's also convert a "x == NULL" to "!x"
per our CodingGuidelines, as we need to change the "if" line anyway.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>