tg: avoid passing '--' options terminator to git cat-file

tg: avoid passing '--' options terminator to git cat-file

Prior to Git version 2.5.0 the `git cat-file` options parsing contained
some special case code that checked the number of arguments provided
before doing general purpose option parsing.

This has the unfortunate side effect of causing an options terminator
option (`--`) to produce an error.

The extra `--` was added to the cat-file command via several commits
that were first released in topgit-0.19.4.  Although this was an
inadvertent change it caused no harm when using Git version 2.5.0
or later.

Strictly speaking, the `--` should have been placed before the object
the command was attempting to get the type of rather than after.

The `-` character is a perfectly valid ref name character and starting
with Git version 2.10.0 the `git cat-file -t` command does accept a
`--allow-unknown-type` option.  Having a 'refs/heads/--allow-unknown-type'
ref name would need the `--` disambiguation if it were provided without
the leading `refs/heads/` part.

Nevertheless, in these cases we will always be passing the output
of `git rev-parse --verify` which will always be a hash value and
never something that can start with a `-` so the disambiguating `--`
will never be required.

Enitrely emove the extraneous `--` from the `git cat-file -t` commands
to restore compatibility with Git versions prior to 2.5.0.

Reported-by: Jonathan Ross Rogers <jrogers@socialserve.com>
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>