Merge branch 'ab/valgrind-fixes'

A bit of test framework fixes with a few fixes to issues found by valgrind. * 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