gbp buildpackage test build with uncommitted changes

[Tong Sun <suntong001@users.sourceforge.net>]

Hi,

I'm new to Debian packaging and gbp buildpackage, and many time I want
to try out my fixes without checking them in, would that be possible
with `gbp buildpackage`?

Because whenever, I have above situation, gbp buildpackage will complains:

- - - - - - - - - - - -
gbp:error: You have uncommitted changes in your source tree:
gbp:error: On branch master
Changes not staged for commit:
. . .
gbp:error: Use --git-ignore-new to ignore.
- - - - - - - - - - - -

Using `--git-ignore-new` will get my changes ignore entirely, and the
build will keep complaining about the same error.

What's the solution? Thx