Sierra 10.12.6 incremental update caught in reboot loop
Download the ComboUpdate 10.12.6 (direct d/l link: macOS Sierra 10.12.6 Combo Update). Open the downloaded dmg and launch the pkg file inside. Reboot when asked.



Alternate solution: clearing the App Store cache often seems to fix the problem.


Quit the App Store
Open Terminal
Run open $TMPDIR../C/com.apple.appstore/
Delete all contents in that folder
Open the App Store again and wait for it to check for updates