feat: add git and history utility functions

- Add branch, fc, and gitup functions
- Update README documentation for new functions and abbreviations
- Remove obsolete push-vim function

functions/branch.fish

@@ -0,0 +1,14 @@
+function branch --description 'Switch to or create a git branch'
+    if not git rev-parse --is-inside-work-tree >/dev/null 2>&1
+        echo "Not a git repo."
+        return 1
+    end
+    
+    # Check if the branch already exists locally
+    if git show-ref --verify --quiet refs/heads/$argv[1]
+        git checkout $argv
+    else
+        # If it doesn't exist, create it
+                git checkout -b $argv
+        end
+end