Fix bundle creation

[axifive]

Fix for added bundle download (#14538). git bundle create can only accept a reference:

2021/09/17 15:37:16 ...archiver/archiver.go:237:func1() [E] Archive &{1 master bundle 8c3a46a6fadf07e24199cf0fbf0fcb973cd5efb8} faild: unable to write archive: exit status 128 - fatal: Refusing to create empty bundle.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@623d2dd). Click here to learn what that means.
The diff coverage is 6.66%.

@@           Coverage Diff           @@
##             main   #17079   +/-   ##
=======================================
  Coverage        ?   45.31%           
=======================================
  Files           ?      774           
  Lines           ?    86983           
  Branches        ?        0           
=======================================
  Hits            ?    39415           
  Misses          ?    41191           
  Partials        ?     6377           

Impacted Files	Coverage Δ
modules/git/repo.go	42.34% <0.00%> (ø)
services/archiver/archiver.go	54.07% <33.33%> (ø)

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[zeripath]

Instead of creating a branch in the real repo this should create a temporary repo with GIT_OBJECT_DIRECTORY set to the main repo and then you create your own branch as appropriate, then the whole temporary repo can be deleted at the end without having to worry about issues like what if a panic is called earlier.

We must not go around creating branches in real users repos. They will appear on the UI - even for a short amount of time - but the current code risks the temporary branch not being deleted at the end if there is an error.

[jolheiser]

Another alternative, maybe we just don't offer bundle downloads for a commit? (If there's no simple way to provide them, that is)
I still personally think the majority of use-cases for bundles are the entire main branch/repo.

[axifive]

@jolheiser the way suggested by @zeripath looks quite simple

[6543]

Why is it kind/breaking?

[axifive]

@6543 added multiple by mistake