Add git.withHTTPCredentials() helper

vars/git.groovy

@@ -0,0 +1,30 @@
+def withHTTPCredentials(String credentialsId, Closure fn) {
+     withCredentials([
+        usernamePassword(
+            credentialsId: credentialsId, 
+            usernameVariable: 'GIT_USERNAME', 
+            passwordVariable: 'GIT_PASSWORD'
+        )
+    ]) {
+        def randomUUID = UUID.randomUUID().toString()
+        def tmpAskPassScript = pwd(tmp:true) + "/${randomUUID}"
+        try {
+            writeFile(
+                file: tmpAskPassScript,
+                text: '''
+                #!/bin/sh
+                case "$1" in
+                Username*) echo $GIT_USERNAME ;;
+                Password*) echo $GIT_PASSWORD ;;
+                esac
+                '''
+            )
+            sh(script: "chmod +x '${tmpAskPassScript}'")
+            withEnv(["GIT_ASKPASS=${tmpAskPassScript}"]) {
+                fn.call()
+            }
+        } finally {
+            sh(script: "rm -f '${tmpAskPassScript}'")
+        }
+    }
+}