remote: fix a leak in query_matches_negative_refspec

remote: fix a leak in query_matches_negative_refspec

In c0192df630 (refspec: add support for negative refspecs, 2020-09-30)
query_matches_negative_refspec() was introduced.

The function was implemented as a two-loop process, where the former
loop accumulates and the latter evaluates.  To accumulate, a string_list
is used.

Within the first loop, there are three cases where a string is added to
the string_list.  Two of them add strings that do not need to be
freed.  But in the third case, the string added is returned by
match_name_with_pattern(), which needs to be freed.

The string_list is initialized with STRING_LIST_INIT_NODUP, i.e.  when
cleared, the strings added are not freed.  Therefore, the string
returned by match_name_with_pattern() is not freed, so we have a leak.

   $ git remote add local .
   $ git update-ref refs/remotes/local/foo HEAD
   $ git branch --track bar local/foo

   Direct leak of 24 byte(s) in 1 object(s) allocated from:
       ... in xrealloc wrapper.c
       ... in strbuf_grow strbuf.c
       ... in strbuf_add strbuf.c
       ... in match_name_with_pattern remote.c
       ... in query_matches_negative_refspec remote.c
       ... in query_refspecs remote.c
       ... in remote_find_tracking remote.c
       ... in find_tracked_branch branch.c
       ... in for_each_remote remote.c
       ... in setup_tracking branch.c
       ... in create_branch branch.c
       ... in cmd_branch builtin/branch.c
       ... in run_builtin git.c

   Direct leak of 24 byte(s) in 1 object(s) allocated from:
       ... in xrealloc wrapper.c
       ... in strbuf_grow strbuf.c
       ... in strbuf_add strbuf.c
       ... in match_name_with_pattern remote.c
       ... in query_matches_negative_refspec remote.c
       ... in query_refspecs remote.c
       ... in remote_find_tracking remote.c
       ... in check_tracking_branch branch.c
       ... in for_each_remote remote.c
       ... in validate_remote_tracking_branch branch.c
       ... in dwim_branch_start branch.c
       ... in create_branch branch.c
       ... in cmd_branch builtin/branch.c
       ... in run_builtin git.c

An interesting point to note is that while string_list_append() is used
in the first two cases described, string_list_append_nodup() is used in
the third.  This seems to indicate an intention to delegate the
responsibility for freeing the string, to the string_list.  As if the
string_list had been initialized with STRING_LIST_INIT_DUP, i.e.  the
strings are strdup()'d when added (except if the "_nodup" API is used)
and freed when cleared.

Switching to STRING_LIST_INIT_DUP fixes the leak and probably is what we
wanted to do originally.  Let's do it.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>