Directory tree hashing should be improved

[jkloetzke]

Whem Bob has to hash a multi-GB workspace it will be stuck for a long time. It also looks like the current algorithm does not handle caching very well if parts of the workspace are updated. The re-hashing still takes very long even though the cache miss rate should be negligible.

• There may be a bug in the cache handling in face of partial workspace changes
• There should be some user feedback if hashing takes very long
• The cache may be improved by compressing common prefixes (like git does)

[mahaase]

I guess, this ticket and #262 are the same?
Could we mark one of them as duplicte?

[jkloetzke]

Actually not. This ticket is about a suspected inefficiency in the algorithm. The other ticket is about the fact that on MSYS on Windows Bob cannot use the right syscalls and is very slow because of that.