stash: refactor private config globals

stash: refactor private config globals

A subsequent commit will access a new config variable in the stash
subcommand implementations, which requires the variables to be declared
before the relevant functions. Prep with a pure refactoring change to
consolidate config-related globals with the rest of the globals.

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Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>