Remove unnecessary argument handler in MergeBase.java

Remove unnecessary argument handler in MergeBase.java

Currently the parsing of command line arguments always results in an
error.

```
fatal: Argument "commit-ish" is required
```

This is caused by the fact that the first method (commit_0) is invoked
twice. (As if the multiValued = true). I couldn't figure out, why that
would be the case, as it's defaults to false and RevCommit handler
doesn't seem to have a logic that would override that either.

In any case, defining Arguments like this is unnecessary as it's
sufficient to define a single ArrayList @Argument that will accumulate
all commits.

Change-Id: Ifd1278b3059f86618db05bfe264dd5c00879d82b