autoproj update fails if git prints warnings to stdout

[FifoIronton]

When updating a package using autoproj, it will fail in a confusing manner if git prints warnings out to stdout.
For a concrete example: I have set git confige core.autocrlf true in my environment, which causes the following output to stdout when adding a file:

$ git add ex.txt
warning: LF will be replaced by CRLF in ex.txt.
The file will have its original line endings in your working directory

Consequently, when running autoproj update in my project, I see the following error:

nick@laptop:/path/buildconf_root/package_dir$ autoproj update
  operating system: ubuntu,debian - 20.04,20.04.1,lts,focal,fossa
  updating bundler
  updating autoproj
  bundler: connected to https://rubygems.org/
  already up-to-date autoproj main configuration
[...]
Command failed
main configuration(/path/buildconf_root/autoproj): failed in import phase
    'git --git-dir /path/buildconf_root/autoproj/.git update-index --add --cacheinfo 100644,warning: LF will be replaced by CRLF in overrides.d/50-versions.yml.,overrides.d/50-versions.yml' returned status 129
    see /path/buildconf_root/install/log/main configuration-import.log for details
    last 1 lines are:

    error: option 'cacheinfo' expects <mode>,<sha1>,<path>

This error can be sidestepped by running git config core.safecrlf false, but this fix was not immediately clear.