Siphash suggested by jrick as a better alternative to murmurhash for this use case. with small fixes from and ok op@

The skip color marks boundary commits and their ancestors. Boundary commits are reachable both via references which we want to exclude from the pack, and via references which we want to include in the pack. We continue processing commit history up to the point we are left with only skip commits on the queue. This can speed up findtwixt() significantly and avoids wrong results produced by the old algorithm which made no distinction between "drop" and "skip". This idea was first implemented by Michael Forney for git9: https://git.9front.org/plan9front/plan9front/2e47badb88312c5c045a8042dc2ef80148e5ab47/commit.html Michael's log message for git9 is reproduced below: git/query: refactor graph painting algorithm (findtwixt, lca) We now keep track of 3 sets during traversal: - keep: commits we've reached from head commits - drop: commits we've reached from tail commits - skip: ancestors of commits in both 'keep' and 'drop' Commits in 'keep' and/or 'drop' may be added later to the 'skip' set if we discover later that they are part of a common subgraph of the head and tail commits. From these sets we can calculate the commits we are interested in: lca commits are those in 'keep' and 'drop', but not in 'skip'. findtwixt commits are those in 'keep', but not in 'drop' or 'skip'. The "LCA" commit returned is a common ancestor such that there are no other common ancestors that can reach that commit. Although there can be multiple commits that meet this criteria, where one is technically lower on the commit-graph than the other, these cases only happen in complex merge arrangements and any choice is likely a decent merge base. Repainting is now done in paint() directly. When we find a boundary commit, we switch our paint color to 'skip'. 'skip' painting does not stop when it hits another color; we continue until we are left with only 'skip' commits on the queue. This fixes several mishandled cases in the current algorithm: 1. If we hit the common subgraph from tail commits first (if the tail commit was newer than the head commit), we ended up traversing the entire commit graph. This is because we couldn't distinguish between 'drop' commits that were part of the common subgraph, and those that were still looking for it. 2. If we traversed through an initial part of the common subgraph from head commits before reaching it from tail commits, these commits were returned from findtwixt even though they were also reachable from tail commits. 3. In the same case as 2, we might end up choosing an incorrect commit as the LCA, which is an ancestor of the real LCA.

This change resulted in a full history walk even when no objects will be added to the pack file. Fix this regression by reverting the change.

The work tree was only held open in order to find its got.conf file since this file could contain a tagger name to use. Read the tagger name earlier. Once the tagger name is known we can close the work tree already.

Fixes failures in our test suite if 'got tag -l | less' is used in the work tree while cmdline tests are running.

reported by naddy

No functional change, the end result is the same.

ok op@

Instead of internally opening and closing the same commit object over and over again, require callers to pass an open commit object in. Avoids an inherent dependency on the commit object cache for reasonable performance. ok op@