While I was looking at the conflict cleanup code, I looked over at the tree cache code, since we clear the tree cache for each entry that gets removed and there is some redundancy there. I made some small tweaks to avoid extra calls to strchr and strlen in a few circumstances.
While C Git has been writing entry count -1 (ie. never other negative numbers) as invalid since day 1, it accepts all negative entry counts as invalid. JGit follows the same rule. libgit2 should also follow, or the index that works with C Git or JGit may someday be rejected by libgit2. Other reimplementations like dulwich and grit have not bothered with parsing or writing tree cache.
The code used to assume that there had to be data after the newline in a tree cache extension entry. This isn't true for a childless invalidated entry if it's the last one, as there won't be any children nor a hash to take up space. Adapt the off-by-one comparison to also work in this case. Fixes #633.