config: fix a leak in git_config_copy_or_rename_section_in_file

config: fix a leak in git_config_copy_or_rename_section_in_file

A branch can have its configuration spread over several configuration
sections.  This situation was already foreseen in 52d59cc645 (branch:
add a --copy (-c) option to go with --move (-m), 2017-06-18) when
"branch -c" was introduced.

Unfortunately, a leak was also introduced:

   $ git branch foo
   $ cat >> .git/config <<EOF
   [branch "foo"]
   	some-key-a = a value
   [branch "foo"]
   	some-key-b = b value
   [branch "foo"]
   	some-key-c = c value
   EOF
   $ git branch -c foo bar

   Direct leak of 130 byte(s) in 2 object(s) allocated from:
       ... in xrealloc wrapper.c
       ... in strbuf_grow strbuf.c
       ... in strbuf_vaddf strbuf.c
       ... in strbuf_addf strbuf.c
       ... in store_create_section config.c
       ... in git_config_copy_or_rename_section_in_file config.c
       ... in git_config_copy_section_in_file config.c
       ... in git_config_copy_section config.c
       ... in copy_or_rename_branch builtin/branch.c
       ... in cmd_branch builtin/branch.c
       ... in run_builtin git.c

Let's fix it.

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