fix(diffr): read revisions with gix lib

[sidkmenon]

Use gix to parse instaed of handrolled parse crap for input git revisions to the diffr cli (e.g. diffr HEAD^1..HEAD).

Text from a clanker:

`diffr X^!` showed `X^` against the working tree. rev-parse accepts `X^!`, so diffr took it for one revision and compared it with the working tree, a different and usually much larger comparison, with no error.

diffr now builds the comparison from the spec gix parses, the way git reads it, instead of splitting `..` and `...` by hand:

- `X^!`: X against its parent; a root commit against the empty tree; a merge is refused, naming its parents.
- `X^-` and `X^-n`: X against its first or nth parent (gix reads these as ranges).
- `a..b` and `a...b`: as before, now from gix's range and merge specs.
- `X^@` and `^X`: refused, since neither names one comparison.
- `--merge-base` and `--cached` keep their rules, checked against the parsed spec.

Tests use git as the oracle: `HEAD^!`, `HEAD^-` and `HEAD^-1` with a dirty working tree, so the old reading would show; `HEAD^@`; and a merge's `^!` against `HEAD^2 HEAD`.

Replaces #1020, which was stacked on the /diffr mod; this is the same change on `main` alone.

https://claude.ai/code/session_01HTHD79i4QdthzVCkDZ26gd