config: allow giving separate author and committer idents

config: allow giving separate author and committer idents

The author.email, author.name, committer.email and committer.name
settings are analogous to the GIT_AUTHOR_* and GIT_COMMITTER_*
environment variables, but for the git config system. This allows them
to be set separately for each repository.

Git supports setting different authorship and committer
information with environment variables. However, environment variables
are set in the shell, so if different authorship and committer
information is needed for different repositories an external tool is
required.

This adds support to git config for author.email, author.name,
committer.email and committer.name  settings so this information
can be set per repository.

Also, it generalizes the fmt_ident function so it can handle author vs
committer identification.

Signed-off-by: William Hubbs <williamh@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>