Support for --stdin-paths in commit, add, etc

Hello,

Hello,

This is a follow-up for previous discussion [1].

In our git UI, we sometimes run into OS commandline length limit when trying to specify a list of pathspecs. For example, this happens when user selects a large set of files and wants to commit them. As a workaround, we split files and issue multiple git commands.

This has multiple disadvantages:
1) Some commands are a lot slower this way. For example, commit-amend
    can take 45 minutes instead of 30 seconds, because git will do a lot
    of unnecessary work.
2) The operation is no longer completed in one "transaction".
3) This makes our code more complicated.

Our suggestion is to change commands such as 'commit', 'add', etc to also work with --stdin-paths. If a command already supports stdin for any purpose, then trying to use both options will return an error.

We're ready to develop the necessary patches to make this possible.

[1] https://public-inbox.org/git/c3be6eff-365b-96b8-16d2-0528612fc1fc@syntevo.com/T/#u