Add support for git 2.38+ automation for performance optimization (scalar)

[gdubicki]

Is your feature request related to a problem? Please describe.

In git 2.38 a new tool has been bundled, scalar, that allows automating enabling the performance optimization features for existing git repos.

It would be nice for tsrc to support this tool to automatically:

• start managing repos with scalar (scalar register),
• update the config of all scalar-manager repositories after git update (scalar reconfigure -a).

Perhaps tsrc could also start using scalar clone instead of git clone which would do a sparse checkout and start to manage the repo with scalar.

Describe the solution you'd like

tsrc could automatically run the above commands if it detects git v. 2.38+. It could detect git version update by storing it in a dot file and then run the config update.

[gdubicki]

Please see https://github.blog/2022-10-03-highlights-from-git-2-38/#a-repository-management-tool-for-large-repositories for some more info.

[gdubicki]

We can already use tsrc foreach to make use of scalar easier:

# -j1 because it writes to a git config file, so it's not a good idea to parallelise
tsrc foreach -j1 scalar register
scalar  reconfigure -a

[dmerejkowsky]

tsrc could automatically run the above commands if it detects git v. 2.38+. It could detect git version update by storing it in a dot file and then run the config update.

Not sure this is a right idea. I think scalar exists as a standalone tool for a good reason.

[dmerejkowsky]

Feel free to add something in the documentation so that users of tsrc learn about this possibility, though :)