improve code style, add comment

Author: kostiakoval

Sources/Get/get().swift

@@ -18,6 +18,7 @@ import Utility
  - Returns: The modules that this manifest requires building
 */
 public func get(manifest: Manifest, manifestParser: (path: String, url: String) throws -> Manifest) throws -> [Package] {
+    // Git 2.0 or higher is required
     if Git.majorVersionNumber < 2 {
         throw Error.ObsoleteGitVersion
     }

Sources/Utility/Git.swift

@@ -63,6 +63,9 @@ public class Git {
             let prefixRange = version.startIndex...version.startIndex.advanced(by: prefix.characters.count)
             version.removeSubrange(prefixRange)
         }
-        return Int(String(version.characters.first!))
+        guard let first = version.characters.first else {
+            return nil
+        }
+        return Int(String(first))
     }
 }