Add missing git icons (#995)

* Add `git-pull-request-create` icon

* Add `git-pull-request-create-arrow` variant

* Improve `git-*` icons metadata

* Optimise `git-pull-request-closed` icon

* Refine `git-fork` icon

…to match [`network`](https://lucide.dev/icons/network)

* Add `git-graph` icon

* Rename `git-commit` to `git-commit-horizontal`

* Add `git-commit-vertical` icon

* Add `git-pull-request-arrow` icon

* Add `git-compare-arrows` icon

icons/git-pull-request-create.json

@@ -0,0 +1,18 @@
+{
+  "$schema": "../icon.schema.json",
+  "contributors": [
+    "danielbayley"
+  ],
+  "tags": [
+    "code",
+    "version control",
+    "open",
+    "plus",
+    "add",
+    "+"
+  ],
+  "categories": [
+    "development",
+    "arrows"
+  ]
+}