plumbing: detect renames by hash and similar content in diff tree

This commit implements the rename detection algorithms used in the
JGit implementation.
Given a list of changes, additions and deletions are extracted and
matched in two ways:
- By exact hash content: all additions and deletions are grouped by
  the content hash and paired with the best match based on the file
  mode and file path. All the files that cannot be paired are kept
  as regular deletions and additions.
- By similar content: a matrix of addition and deletion pairs with
  all possible combinations is created and scored by how similar the
  content is between both files as well as how similar the file path
  is. The pairs with the best score and whose score is equal or
  greater than a threshold are paired and turned into a rename.
  All the files that cannot be paired are kept as regular deletions
  and additions.

DiffTree and DiffTreeContext will not return the changes with renames
detected for compatibility reasons, although this will change in v6
so that detecting renames is the default behaviour.
A new function DiffTreeWithOptions has been added to configure the
parameters for the rename detection to control the score threshold,
the limit of renames and whether to use similar content detection in
the detection.

More information:

- https://github.com/eclipse/jgit/blob/master/org.eclipse.jgit/src/org/eclipse/jgit/diff/RenameDetector.java
- https://github.com/eclipse/jgit/blob/master/org.eclipse.jgit/src/org/eclipse/jgit/diff/SimilarityRenameDetector.java
- https://github.com/eclipse/jgit/blob/master/org.eclipse.jgit/src/org/eclipse/jgit/diff/SimilarityIndex.java

Signed-off-by: Miguel Molina <miguel@erizocosmi.co>

1 files changed, 3 insertions, 1 deletions

diff --git a/plumbing/object/change.go b/plumbing/object/change.go
index 4474905..c9d1615 100644
--- a/plumbing/object/change.go
+++ b/plumbing/object/change.go
@@ -18,7 +18,7 @@ type Change struct {
 	To   ChangeEntry
 }
 
-var empty = ChangeEntry{}
+var empty ChangeEntry
 
 // Action returns the kind of action represented by the change, an
 // insertion, a deletion or a modification.
@@ -27,9 +27,11 @@ func (c *Change) Action() (merkletrie.Action, error) {
 		return merkletrie.Action(0),
 			fmt.Errorf("malformed change: empty from and to")
 	}
+
 	if c.From == empty {
 		return merkletrie.Insert, nil
 	}
+
 	if c.To == empty {
 		return merkletrie.Delete, nil
 	}