perf(tar): cache the parse result of GNU tar help

[akinomyoga]

In my setup, the tar completion has been somehow significantly slower than the other completions. It turned out that the help parsing, which is performed for every call of the tar completion, has been the bottleneck. This PR introduces a cache for the parse result, which significantly improves the performance of the tar completion.

The main change is 5a85108. There are other refactoring commits, but those changes are independent.

[scop]

LGTM. Separate definition and initialization of variables seems to be something we don't generally do, I wonder if there's a reason for not doing them in one go here as well? Anyway, approved as is or with initialization moved to definitions.

[akinomyoga]

I wonder if there's a reason for not doing them in one go here as well?

Yeah, I also thought about that option. I updated the commit that way. In commit c1e96b7, we initialize _comp_cmd_gtar__parse_help__result when loading the file completions/tar (rather than the first call of _comp_cmd_gtar__parse_help). Now the function _comp_cmd_gtar__parse_help just evaluates "$_comp_cmd_gtar__parse_help__result", so I renamed it to _comp_cmd_gtar__parse_help_load.