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: 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>
git-cvsimport-3: add a version check for new features

Instead of failing with an obscure cvsps error if a feature that is
required is not supported, add a way to check for a given version before
invoking cvsps.

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: pass "-i" to cvsps for incremental imports

This flag is required for cvsps to output the correct incremental
fast-import stream, so git-cvsimport-3 will fail if the available cvsps
does not support it.

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: remove an unused variable

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