[CHORE] Drop go-git support

See https://codeberg.org/forgejo/discussions/issues/164 for the
rationale and discussion of this change.

Everything related to the `go-git` dependency is dropped (Only a single
instance is left in a test file to test for an XSS, it requires crafting
an commit that Git itself refuses to craft). `_gogit` files have
been removed entirely, `go:build: !gogit` is removed, `XXX_nogogit.go` files
either have been renamed or had their code being merged into the
`XXX.go` file.

modules/git/tree_blob.go

@@ -5,7 +5,48 @@
 
 package git
 
-import "strings"
+import (
+	"path"
+	"strings"
+)
+
+// GetTreeEntryByPath get the tree entries according the sub dir
+func (t *Tree) GetTreeEntryByPath(relpath string) (*TreeEntry, error) {
+	if len(relpath) == 0 {
+		return &TreeEntry{
+			ptree:     t,
+			ID:        t.ID,
+			name:      "",
+			fullName:  "",
+			entryMode: EntryModeTree,
+		}, nil
+	}
+
+	// FIXME: This should probably use git cat-file --batch to be a bit more efficient
+	relpath = path.Clean(relpath)
+	parts := strings.Split(relpath, "/")
+	var err error
+	tree := t
+	for i, name := range parts {
+		if i == len(parts)-1 {
+			entries, err := tree.ListEntries()
+			if err != nil {
+				return nil, err
+			}
+			for _, v := range entries {
+				if v.Name() == name {
+					return v, nil
+				}
+			}
+		} else {
+			tree, err = tree.SubTree(name)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+	return nil, ErrNotExist{"", relpath}
+}
 
 // GetBlobByPath get the blob object according the path
 func (t *Tree) GetBlobByPath(relpath string) (*Blob, error) {