Fix for a tiny bug where `arc land` will fail if a branch has the same name a file path in the root of the repo.

(In fbcode and www at least, the only places I tried) if a
branch happens to be named the same as a file path (from root) (like if
a branch is called `spec` and there is a directory in `www` called
`spec`) then `arc land` will fail with git complaining that the argument
(to `git log ...`) is ambiguous as it could refer to both a path and a
revision; so this diff adds a `--` to the end so that git knows that
both are revisions and not paths.

Test Plan: Try and land something from www (I did, see D752219) where
the branch is named `spec`.

Reviewers:

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Task ID: #

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