tg: better accomodate user.useConfigOnly=true

tg: better accomodate user.useConfigOnly=true

While it is true that "git var GIT_AUTHOR_IDENT" and/or
"git var GIT_COMMITTER_IDENT" can actually fail in some cases
when user.useConfigOnly is not true, it's somewhat unlikely.

However, with the advent of user.useConfigOnly=true it's
much more likely that a situation will be encountered where
"git var GIT_AUTHOR_IDENT" and/or "git var GIT_COMMITTER_IDENT"
will fail.

It's very inconvenient when this happens in the middle of some
TopGit operation.  It's much better if the failure takes place
at the beginning of such an operation before any data has been
enetered or any changes have been made.

Therefore, run "git var GIT_AUTHOR_IDENT" and
"git var GIT_COMMITTER_IDENT" at various strategic locations
(and exit if either fails) so that any issues can be corrected
before TopGit starts to make any changes.

This provides a much improved user experience.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>