Use git config file for committer name and email info

Use git config file for committer name and email info

This starts using the "user.name" and "user.email" config variables if
they exist as the default name and email when committing.  This means
that you don't have to use the GIT_COMMITTER_EMAIL environment variable
to override your email - you can just edit the config file instead.

The patch looks bigger than it is because it makes the default name and
email information non-static and renames it appropriately.  And it moves
the common git environment variables into a new library file, so that
you can link against libgit.a and get the git environment without having
to link in zlib and libcrypt.

In short, most of it is renaming and moving, the real change core is
just a few new lines in "git_default_config()" that copies the user
config values to the new base.

It also changes "git-var -l" to list the config variables.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>