built-ins: use free() not UNLEAK() if trivial, rm dead code

For a lot of uses of UNLEAK() it would be quite tricky to release the
memory involved, or we're missing the relevant *_(release|clear)()
functions. But in these cases we have them already, and can just
invoke them on the variable(s) involved, instead of UNLEAK().

For "builtin/worktree.c" the UNLEAK() was also added in [1], but the
struct member it's unleaking was removed in [2]. The only non-"int"
member of that structure is "const char *keep_locked", which comes to
us via "argv" or a string literal[3].

We have good visibility via the compiler and
tooling (e.g. SANITIZE=address) on bad free()-ing, but none on
UNLEAK() we don't need anymore. So let's prefer releasing the memory
when it's easy.

For "bugreport", "worktree" and "config" we need to start using a "ret
= ..." return pattern. For "builtin/bugreport.c" these UNLEAK() were
added in [4], and for "builtin/config.c" in [1].

For "config" the code seen here was the only user of the "value"
variable. For "ACTION_{RENAME,REMOVE}_SECTION" we need to be sure to
return the right exit code in the cases where we were relying on
falling through to the top-level.

I think there's still a use-case for UNLEAK(), but hat it's changed
since then. Using it so that "we can see the real leaks" is
counter-productive in these cases.

It's more useful to have UNLEAK() be a marker of the remaining odd
cases where it's hard to free() the memory for whatever reason. With
this change less than 20 of them remain in-tree.

1. 0e5bba53af (add UNLEAK annotation for reducing leak false
   positives, 2017-09-08)
2. d861d34a6e (worktree: remove extra members from struct add_opts,
   2018-04-24)
3. 0db4961c49 (worktree: teach `add` to accept --reason <string> with
  --lock, 2021-07-15)
4. 0e5bba53af and 00d8c31105 (commit: fix "author_ident" leak,
   2022-05-12).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

builtin/commit.c

@@ -1874,8 +1874,8 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	apply_autostash(git_path_merge_autostash(the_repository));
 
 cleanup:
-	UNLEAK(author_ident);
-	UNLEAK(err);
-	UNLEAK(sb);
+	strbuf_release(&author_ident);
+	strbuf_release(&err);
+	strbuf_release(&sb);
 	return ret;
 }