completion: clear cached --options when sourcing the completion script

The established way to update the completion script in an already running shell is to simply source it again: this brings in any new --options and features, and clears caching variables. E.g. it clears the variables caching the list of (all|porcelain) git commands, so when they are later lazy-initialized again, then they will list and cache any newly installed commmands as well. Unfortunately, since d401f3debc (git-completion.bash: introduce __gitcomp_builtin, 2018-02-09) and subsequent patches this doesn't work for a lot of git commands' options. To eliminate a lot of hard-to-maintain hard-coded lists of options, those commits changed the completion script to use a bunch of programmatically created and lazy-initialized variables to cache the options of those builtin porcelain commands that use parse-options. These variables are not cleared upon sourcing the completion script, therefore they continue caching the old lists of options, even when some commands recently learned new options or when deprecated options were removed. Always 'unset' these variables caching the options of builtin commands when sourcing the completion script. Redirect 'unset's stderr to /dev/null, because ZSH's 'unset' complains if it's invoked without any arguments, i.e. no variables caching builtin's options are set. This can happen, if someone were to source the completion script twice without completing any --options in between. Bash stays silent in this case. Add tests to ensure that these variables are indeed cleared when the completion script is sourced; not just the variables caching options, but all other caching variables, i.e. the variables caching commands, porcelain commands and merge strategies as well. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>