Improve performance with large numbers of untracked or modified files (#3919)

- **PR Description**
**BuildTreeFromFiles** used a linear complexity lookup to find if the
children has already been added. Use maps to get constant time lookup
for children.

For the test scenario in #3798 (90000 untracked files) this reduces the
time of BuildTreeFromFiles from something like 10s down to about 30ms on
my machine.

Fixes #3798.

- **Please check if the PR fulfills these requirements**

* [x] Cheatsheets are up-to-date (run `go generate ./...`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [ ] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [ ] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [ ] If a new UserConfig entry was added, make sure it can be
hot-reloaded (see
[here](https://github.com/jesseduffield/lazygit/blob/master/docs/dev/Codebase_Guide.md#using-userconfig))
* [ ] Docs have been updated if necessary
* [x] You've read through your own file changes for silly mistakes etc

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pkg/gui/filetree/build_tree.go

@@ -10,6 +10,8 @@ import (
 func BuildTreeFromFiles(files []*models.File) *Node[models.File] {
 	root := &Node[models.File]{}
 
+	childrenMapsByNode := make(map[*Node[models.File]]map[string]*Node[models.File])
+
 	var curr *Node[models.File]
 	for _, file := range files {
 		splitPath := split(file.Name)
@@ -23,11 +25,19 @@ func BuildTreeFromFiles(files []*models.File) *Node[models.File] {
 			}
 
 			path := join(splitPath[:i+1])
-			for _, existingChild := range curr.Children {
-				if existingChild.Path == path {
-					curr = existingChild
-					continue outer
-				}
+
+			var currNodeChildrenMap map[string]*Node[models.File]
+			var isCurrNodeMapped bool
+
+			if currNodeChildrenMap, isCurrNodeMapped = childrenMapsByNode[curr]; !isCurrNodeMapped {
+				currNodeChildrenMap = make(map[string]*Node[models.File])
+				childrenMapsByNode[curr] = currNodeChildrenMap
+			}
+
+			child, doesCurrNodeHaveChildAlready := currNodeChildrenMap[path]
+			if doesCurrNodeHaveChildAlready {
+				curr = child
+				continue outer
 			}
 
 			newChild := &Node[models.File]{
@@ -36,6 +46,8 @@ func BuildTreeFromFiles(files []*models.File) *Node[models.File] {
 			}
 			curr.Children = append(curr.Children, newChild)
 
+			currNodeChildrenMap[path] = newChild
+
 			curr = newChild
 		}
 	}