keep gitignore & gitattributes on upgrade/rename

[guyhughes]

fixes #37, further to #54 -- I feel that the user expects to keep the .gitignore.d/foo and .gitattributes.d/foo files in effect when renaming repo foo to bar. Currently upgrade() abandons the foo files and updates .git to point to bar files.

I've implemented renaming the files with mv in upgrade() so that even if core.excludesfile or core.attributesfile points elsewhere, they can be moved into the recommended location.

mv will exit with an error instead of overwriting any file, and I've handled showing that to the user.

I've only played with this a bit. Open for improvements.

[alerque]

Hey thanks for taking the time to contribute this. I did some cleanup work to the coding style and tweaked some shell scripting issues. I haven't had a chance to put it through any paces or really figure out if the approach is right, but I can't push to this PR (it was opened under GitHub's legacy system, you might have a check box to enable maintainers to modify the PR) but wanted to note that I stashed the WIP on this in this branch.

[alerque]

See also extra discussion in #163 and particular the alternative implementation in this branch. That should be compared as part of reviewing this.

[alerque]

Hey @guyhughes just a gentle ping if you're around and can check for that "allow maintainers..." checkbox in the sidebar that would facilitate moving this along finally.