git: add debug logging when there's a mismatch in the cached heads list
The dag rebuild can be expensive, so let's try and avoid bugs where it
transparently rebuilds all the time for no reason. This would have
prevented the issue fixed in D8622.
Differential Revision: https://phab.mercurial-scm.org/D8625
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.