Merge branch 'tb/fix-persistent-shallow' into master

When "fetch.writeCommitGraph" configuration is set in a shallow
repository and a fetch moves the shallow boundary, we wrote out
broken commit-graph files that do not match the reality, which has
been corrected.

* tb/fix-persistent-shallow:
  commit.c: don't persist substituted parents when unshallowing

diff --cc commit-graph.c
Simple merge

diff --cc commit.c
Simple merge

diff --cc object.h
Simple merge

diff --cc t/t5537-fetch-shallow.sh
Simple merge