Move git command as a standalone package so that it's easier to move the repositories to another machine

[lunny]

As a step to support distributed Git data storage, I make an abstract git service and command. Currently, just LocalService and LocalCommand has been implemented in this PR. But I think it's not very difficult to implement distributed one after this merged.

A new standalone command or a Gitea subcommand could be written to be as a git command service(http/tcp/grpc?). So the git commands could be sent to a remote service and retrieve the result. But this will be in future PRs.

[wxiaoguang]

If the git package should be refactored to support local/remote, it might be like this:

git.Command (interface)
git.localcmd.Command
git.remotecmd.Command

However, it seems too early to introduce the local/remote package, is there any real requirement at the moment or a detailed design? If there is no real requirement and design, what is done now might be wrong in future.

So the package level refactoring should be made after there is clear requirement and design.

[wxiaoguang]

Replacing process.GetManager().Exec with git.Command.Run is fine, and RunStdString could be used to simplify the code.

[codecov-commenter]

Codecov Report

Merging #18147 (03c1045) into main (6171ea7) will increase coverage by 0.01%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##             main   #18147      +/-   ##
==========================================
+ Coverage   47.28%   47.30%   +0.01%     
==========================================
  Files         957      959       +2     
  Lines      133374   133557     +183     
==========================================
+ Hits        63067    63179     +112     
- Misses      62633    62713      +80     
+ Partials     7674     7665       -9     

Impacted Files	Coverage Δ
cmd/serv.go	2.32% <0.00%> (-0.02%)	⬇️
models/action.go	48.92% <0.00%> (ø)
models/db/engine.go	33.55% <ø> (ø)
modules/context/repo.go	51.42% <0.00%> (ø)
modules/git/repo_attribute.go	68.51% <ø> (ø)
modules/markup/external/external.go	1.31% <0.00%> (-0.04%)	⬇️
modules/setting/repository.go	55.71% <ø> (ø)
routers/api/v1/repo/release_attachment.go	0.00% <0.00%> (ø)
routers/install/install.go	1.77% <0.00%> (-0.01%)	⬇️
services/mailer/mailer.go	29.79% <0.00%> (-0.25%)	⬇️
... and 55 more

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[lunny]

Since this PR opened, the command has been changed a lot. Just close it.

[6543]

still it should be done at some point ... to make it possible to have different back-ends for this, I'll put that on my "long term todos"