I have my project + RestKit submodule. Error appeared when I changed RestKit settings. I added support armv6 and armv7 architecture.
git add .
git add -u
g
You are apparently dealing with a submodule, so you should use the submodule workflow :
# Modification on RestKit, for instance :
cd RestKit
git add .
git commit -m "Support for armv6 & armv7"
cd ..
# RestKit submodule up-to-date, now update your project
git add RestKit
git commit -m "RestKit submodule updated"
You can find more information here.
Also : Git Book on Submodules