implement importChanges optimisaton (not used yet)

For simplicity, I've not tried to make it handle History yet, so when
there is a history, a full import will still be done. Probably the right
way to handle history is to first diff from the current tree to the last
imported tree. Then, diff from the current tree to each of the
historical trees, and recurse through the history diffing from child tree
to parent tree.

I don't think that will need a record of the previously imported
historical trees, and so Logs.Import doesn't store them. Although I did
leave room for future expansion in that log just in case.

Next step will be to change importTree to importChanges and modify
recordImportTree et all to handle it, by using adjustTree.

Sponsored-by: Brett Eisenberg on Patreon

Git/FilePath.hs

@@ -46,7 +46,7 @@ instance NFData TopFilePath
 {- A file in a branch or other treeish. -}
 data BranchFilePath = BranchFilePath Ref TopFilePath
 	deriving (Show, Eq, Ord)
-
+ 
 {- Git uses the branch:file form to refer to a BranchFilePath -}
 descBranchFilePath :: BranchFilePath -> StringContainingQuotedPath
 descBranchFilePath (BranchFilePath b f) =