Merge branch 'ab/valgrind-fixes' into maint

A bit of test framework fixes with a few fixes to issues found by
valgrind.
source: <redacted>

* ab/valgrind-fixes:
  commit-graph.c: don't assume that stat() succeeds
  object-file: fix a unpack_loose_header() regression in 3b6a8db3b03
  log test: skip a failing mkstemp() test under valgrind
  tests: using custom GIT_EXEC_PATH breaks --valgrind tests