t6020: test for duplicate refnames in bundle creation

t6020: test for duplicate refnames in bundle creation

The commit b2a6d1c686 (bundle: allow the same ref to be given more than
once, 2009-01-17) added functionality to detect and remove duplicate
refnames from being added during bundle creation. This ensured that
clones created from such bundles wouldn't barf about duplicate refnames.

The following commit will add some optimizations to make this check
faster, but before doing that, it would be optimal to add tests to
capture the current behavior.

Add tests to capture duplicate refnames provided by the user during
bundle creation. This can be a combination of:

  - refnames directly provided by the user.
  - refname duplicate by using the '--all' flag alongside manual
    references being provided.
  - exclusion criteria provided via a refname "main^!".
  - short forms of refnames provided, "main" vs "refs/heads/main".

Note that currently duplicates due to usage of short and long forms goes
undetected. This should be fixed with the optimizations made in the next
commit.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>