[BUG] DO NOT use short hash for commit versions (DOS attack)

[Leptopoda]

Before creating a bug report please make check the following

• You have read our FAQ
• If you have used flutter. Please install correctly, run pub cache repair. Close the terminal and try again.
• If you are on Windows. Make sure you are running the terminal as administrator or with developer permissions.
• Run fvm doctor if possible and add the output to the issue.

Describe the bug
Using a commit version of flutter will specify the short hash in the .fvmrc (and vscode config) files.
This can easily be DOSed as commit hashes are shared between forks of the repo.
You can read more about it in: https://blog.teddykatz.com/2019/11/12/github-actions-dos.html

To Reproduce
run fvm use 6d04a162109d07876230709adf4013db113b16a3
see the content of .fvmrc

{
  "flutter": "6d04a16210"
}

Expected behavior
The full commit hash is used in the config file.
Also when just the short commit was specified in the first place

Logs
N.A.

Desktop (please complete the following information):
N.A.

Additional context
Short hashes should only ever be used to display a nice version to the user.
Never to refer to any particular commit (especially on GitHub).

[leoafarias]

@Leptopoda, thank you for opening this issue. Do you think you would be able to create a PR addressing this? I believe this should be fairly straightforward, but I am still trying to catch up on the issues and PRs in fvm.

[Leptopoda]

I'm fairly caught up in work myself, so don't count on me.
I'll take a shot at it once I have more time.