fix(git): correct blob-show classification for :^ pathspec and separated flag operands

B1: `:^` is an exact synonym of `:!` (exclude magic) and is a pathspec, not
a blob, so exclude it from `is_blob_show_arg` — `git show <rev> :^dir` now
routes to the commit-diff path instead of a raw blob dump.

B2: only git show's first positional object can be a `<rev>:<path>` blob.
`show_positionals` walks the args with git's own flag/value grammar
(`consumes_next_token_as_value`), so a value-taking flag's separate operand
(`-S 'url:1'`, `-L 1,2:file`) is skipped rather than misread as a blob and
truncated to 8 KiB. This also resolves the blob/commit-mix case.