hg: semantic fix for "show last" alias

`hg log --limit 1` is basically equivalent to `hg log tip` -- tip is the
newest revision, not necessarily the current one, e.g. if you've checked
out an older rev. `git show` with no explicit revision argument acts on
the *current* rev, so my `last` alias for hg should more appropriately
use `--rev .`
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