Support `@` as alias of `HEAD`

[L3viathan]

Git allows specifying the current head as either HEAD or @. git-absorb apparently doesn't:

$ git-absorb --base @~20
Sep 26 09:37:45.336 CRIT absorb failed, err: revspec '@' not found; class=Reference (4); code=NotFound (-3)
$ git-absorb --base HEAD~20
# works

[tummychow]

https://github.com/libgit2/libgit2/blob/9f3406163bf85eec75c6095154a27d3be7448b45/src/libgit2/revparse.c#L803-L825