Refactor editor upload, update and delete to use git plumbing and add LFS support

[zeripath]

When uploading files using the GUI, Gitea currently does:

a) A full clone - Including the whole object DB
b) A full branch checkout - every file
c) Copies the uploaded file to this cloned repository
d) Add/stages the changes
e) Commits
f) Pushes to the real repository
g) Cleans up the cloned repository - deleting the whole object db and checkout.

This is extremely inefficient.

This PR takes a different approach and, instead of using the git porcelain commands (git add and the like), uses the plumbing commands to (hopefully) significantly speed up this process. See my comments in #601 (#601 (comment)).

The basics of this code was in #5621, however, there has been significant refactoring with code moved out of models.Repository into a module called uploader. LFS support has been added, with transparent recognition of whether a file should be added to the LFS through checking the git attributes.

Issues

• Currently my approach is to use the git-cli commands which can/could be migrated to go-git commands later. Some of this code could and probably should be moved into the gitea git subproject.
• There are inefficiencies in the LFS code which could be improved - for example, when files are uploaded to the server, their sha1dcsum and sha256sum should be created at that point rather than read the whole file again - and the LFS ContentStore will currently regenerate the sha256sum and copy the file again.
• I don't set ORIGINAL_SSH_COMMAND so the push hooks may not actually run. This is to stay in keeping with the current implementation - however, could be very easily set.
• It's not possible to update LFS files, although it is possible to create new text files that will become LFS files.

Will fix #5600. Will fix #4778. Will fix #3557. Will fix #5727.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@baffea1). Click here to learn what that means.
The diff coverage is 32.51%.

@@            Coverage Diff            @@
##             master    #5702   +/-   ##
=========================================
  Coverage          ?   38.82%           
=========================================
  Files             ?      344           
  Lines             ?    49355           
  Branches          ?        0           
=========================================
  Hits              ?    19163           
  Misses            ?    27428           
  Partials          ?     2764

Impacted Files	Coverage Δ
modules/uploader/diff.go	0% <0%> (ø)
modules/uploader/delete.go	0% <0%> (ø)
models/lfs.go	32.72% <0%> (ø)
models/upload.go	0% <0%> (ø)
modules/uploader/upload.go	0% <0%> (ø)
routers/repo/editor.go	28.67% <26.31%> (ø)
modules/uploader/update.go	35.71% <35.71%> (ø)
models/repo_branch.go	58.27% <56%> (ø)
modules/uploader/repo.go	67.04% <67.04%> (ø)

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[zeripath]

As previously described in #5621 this provides around a 10x speed up on the current implementation even for very small repositories.

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Speed ups for large repositories should be considerably better.

[zeripath]

@lunny needs your review.

[zeripath]

Ping @lunny

[zeripath]

Make lg TM work

[richmahn]

Any way to retain commit history when a file is renamed in the GUI? Now it deletes the old file, makes a new file. That isn't the behavior of GitHub when you change the name or path of an existing file.

[lafriks]

@richmahn please submit issue for this