[RFC] Add way to make Git credentials accessible from clean/smudge filter

Hi,

we just implemented the first "real-world" user of the new clean/smudge "filter protocol" interface (see "convert: add filter.<driver>.process option" edcc858 for details) and the results are fantastic. Filtering 12,000 files in my artificial test repo is more than 60x faster (depending on the platform). On Windows that means the clean/smudge operations runs in 57 seconds instead of 55 minutes [1]!

I have a number of ideas to improve the protocol even further and I am seeking early feedback on the following - possibly most controversial - idea:

Some filters might want to perform additional network interactions and these filters would like to use the Git credentials to perform these actions. If such a filter is configured with "offerCredentials = true" then the filter could request the current Git credentials via the "filter-protocol".

A configuration could look like this:
------------------------
[filter "myfilter"]
    process = my-filter-process
    required = true
    offerCredentials = true
------------------------

The default would, of course, be "offerCredentials = false".

I haven't looked at an implemenation approach at all. I wonder if this could be OK from a conceptional point of view or if there are obvious security problems that I am missing.

Thanks, Lars

[1] https://github.com/github/git-lfs/pull/1617#issuecomment-259411850