git-cvsimport-3: read configuration from git config

git-cvsimport-2 supports reading options from the Git repository into
which it was importing.  Make git-cvsimport-3 do the same.

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: overwrite CVS revision map

The current version of cvsps-3 appears to write the entire revision map
rather than just the delta since the previous run.  To handle this
correctly, open the git-cvsimport revision map file as "w" instead of
"a".
git-cvsimport-3: change subprocess helpers to passthrough args

This allows us to use the do_or_die() and capture_or_die() helpers for
all invocations of subprocesses, even when we need to specify custom
stdin or stdout.

Signed-off-by: John Keeping <john@keeping.me.uk>