Re: [PATCH v2] maintenance --unregister: fix uninit'd data use & -Wdeclaration-after-statement

> Since (maintenance: add option to register in a specific config,
> 2022-11-09) we've been unable to build with "DEVELOPER=1" without
> "DEVOPTS=no-error", as the added code triggers a
> "-Wdeclaration-after-statement" warning.
>
> And worse than that, the data handed to git_configset_clear() is
> uninitialized, as can be spotted with e.g.:
>
> 	./t7900-maintenance.sh -vixd --run=23 --valgrind
> 	[...]
> 	+ git maintenance unregister --force
> 	Conditional jump or move depends on uninitialised value(s)
> 	   at 0x6B5F1E: git_configset_clear (config.c:2367)
> 	   by 0x4BA64E: maintenance_unregister (gc.c:1619)
> 	   by 0x4BD278: cmd_maintenance (gc.c:2650)
> 	   by 0x409905: run_builtin (git.c:466)
> 	   by 0x40A21C: handle_builtin (git.c:721)
> 	   by 0x40A58E: run_argv (git.c:788)
> 	   by 0x40AF68: cmd_main (git.c:926)
> 	   by 0x5D39FE: main (common-main.c:57)
> 	 Uninitialised value was created by a stack allocation
> 	   at 0x4BA22C: maintenance_unregister (gc.c:1557)
>
> Let's fix both of these issues, and also move the scope of the
> variable to the "if" statement it's used in, to make it obvious where
> it's used.
>
> Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>