classes/buildhistory: make a single commit per build

classes/buildhistory: make a single commit per build

Way back in OE-Core commit fba198ac7efe476a25c5761878ef2fcee97bf9f1 in
2012 we split committing to the repository, making a commit per
top-level directory. However, as we add more information it becomes
harder to see which commits belong to which build. Switch back to a
single commit per build to keep the history tidier.

To address the original concern, if you do want to see just the changes
for a particular subdirectory, git can filter that for you - just
specify that subdirectory as the last parameter on the git show / git
diff command line and that's all you will see.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>