[Downloader] Repos set with tag as branch break `[p]cog update`

# Command bugs

#### Command name

`[p]cog update`

#### What cog is this command from?

Downloader

#### What were you expecting to happen?

Cog update to proceed without error or at least to error gracefully.

#### What actually happened?

Got unhandled exception:
```
Exception in command 'cog update'
Traceback (most recent call last):
File "/home/ubuntu/red-venv/lib/python3.8/site-packages/discord/ext/commands/core.py", line 79, in wrapped
ret = await coro(*args, **kwargs)
File "/home/ubuntu/red-venv/lib/python3.8/site-packages/redbot/cogs/downloader/downloader.py", line 841, in _cog_update
await self._cog_update_logic(ctx, cogs=cogs)
File "/home/ubuntu/red-venv/lib/python3.8/site-packages/redbot/cogs/downloader/downloader.py", line 915, in _cog_update_logic
cogs_to_check, check_failed = await self._get_cogs_to_check(repos=repos, cogs=cogs)
File "/home/ubuntu/red-venv/lib/python3.8/site-packages/redbot/cogs/downloader/downloader.py", line 1159, in _get_cogs_to_check
__, failed = await self._repo_manager.update_repos()
File "/home/ubuntu/red-venv/lib/python3.8/site-packages/redbot/cogs/downloader/repo_manager.py", line 1175, in update_repos
updated_repo, (old, new) = await self.update_repo(repo.name)
File "/home/ubuntu/red-venv/lib/python3.8/site-packages/redbot/cogs/downloader/repo_manager.py", line 1142, in update_repo
old, new = await repo.update()
File "/home/ubuntu/red-venv/lib/python3.8/site-packages/redbot/cogs/downloader/repo_manager.py", line 805, in update
await self.hard_reset()
File "/home/ubuntu/red-venv/lib/python3.8/site-packages/redbot/cogs/downloader/repo_manager.py", line 785, in hard_reset
raise errors.HardResetError(
redbot.cogs.downloader.errors.HardResetError: Some error occurred when trying to execute a hard reset on the repo at the following path: /home/ubuntu/red-local/cogs/RepoManager/repos/jack-rlstats31
```

#### How can we reproduce this issue?

1. Add repo with a tag as a branch, i.e:
`[p]repo add jackcogs-test https://github.com/jack1142/JackCogs rlstats-3.1`
2. Run `[p]cog update`