Delete local branch when repo branch is deleted

[jolheiser]

Fixes #6490

Currently, when creating a branch from the web UI (not a commit, directly from the branch dropdown), Gitea creates the branch in its local cache and pushes it to the real repo.

When the branch is deleted, it was not being deleted from the local cache and if a branch of the same name is created, Gitea panics because the local repo already has a branch with that name.

Since the Gitea's tmp directory is deleted on startup, the only (current) way to clear the local repo cache is to restart Gitea.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@49b2f45). Click here to learn what that means.
The diff coverage is 25%.

@@            Coverage Diff            @@
##             master    #6497   +/-   ##
=========================================
  Coverage          ?   40.41%           
=========================================
  Files             ?      404           
  Lines             ?    54140           
  Branches          ?        0           
=========================================
  Hits              ?    21883           
  Misses            ?    29240           
  Partials          ?     3017

Impacted Files	Coverage Δ
routers/repo/branch.go	56.9% <0%> (ø)
models/repo_branch.go	54.97% <30%> (ø)

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[zeripath]

The local cache needs to go. Do we even need it still?

I cannot believe that we're actually requiring a full clone to just do a branch creation - particularly a full clone we keep around.

Can't we follow the technique in

https://github.com/go-gitea/gitea/blob/master/modules/uploader/update.go

Where we just do a thin clone, create an index from the appropriate old branch and then push it as the new branch. We probably don't even need an index - We could just push our HEAD to the new branch.

In fact I wouldn't be surprised if we couldn't just do a push from an empty place to create a new branch. Would have to look at the documentation for git push.

A branch is simply a file with a hash to a tree object. It's nothing magical. The only benefit with pushing is that you make sure hooks etc will run.

[jolheiser]

If I do follow how the uploader does it, should I take that code and put it somewhere else (perhaps a new module, or in util) that can be re-used when we need to make a "temp" repo that gets cleaned up after use?

I could just use the existing code, but it would seem odd to use the uploader package when referencing temp repos.

[zeripath]

I'm gonna approve as this is a bug fix, but I'm gonna look at removing this unnecessary cloning from the codebase.

[zeripath]

Take a look at: #6505

[ngourdon]

@zeripath Even if it's out of scope of this PR, yes you can push a new branch without checking it out.

The form of the git push command which will permit to do so is
git push <repository> <refspec>
repository can be a name like "origin" but may also be a path like "/path/to/gitea-repositories/username/repo.git".

From the directory of the gitea bare repository (/path/to/gitea-repositories/username/repo.git) you can do
git push . refs/heads/existing-branch:refs/heads/new-branch
or from a sha1
git push . sha1:refs/heads/new-branch

This will trigger git hooks

[jolheiser]

@zeripath Once your PR is merged I will look at submitting a new PR implementing those changes in this area. (Unless your PR just removes these changes as part of the conversion)