Fix git config bug (credential section duplication).

a git config --unset credential.helper followed by
git config credential.helper results in
    [credential]
    [credential]
      helper = ...
Repeat that often enough and the file will be too big.

Fix pull_external script by removeing the credential section.
git config --remove-section credential
remove-section credential instead of unset.

See commit 19af2ae: unset triggers a duplication of the section
when the next git config is applied for that same section.