Merge branch 'ps/commit-graph-overflow-fix'

Fix a regression in writing the commit-graph where commits with dates
exceeding 34 bits (beyond year 2514) could cause an underflow and
crash Git during the generation data overflow chunk writing.

* ps/commit-graph-overflow-fix:
  commit-graph: fix writing generations with dates exceeding 34 bits

diff --cc commit-graph.c
Simple merge