Tentative fix to git-commit-script

Tentative fix to git-commit-script

The recent change to give the multiple commit message source was not
carrying over the authorship information from -C/-c commits correctly.
The export of the environment variable happens only in the subprocess,
not the main process that eventually runs git-commit-tree.

The right fix might be to teach git-commit-script to grok the From:
and Date: lines at the beginning of the commit message just like
git-applymbox knows how, but this has to do until that enhancement
happens.

Signed-off-by: Junio C Hamano <junkio@cox.net>