fix(git-cz.js,staging.js): check for staged files before running prompt

Check for staged files before running the prompt. Running the `git-cz` command with no files staged
reports "No files added to staging". Preserve the functionality of `git -a` (--all) flag - `git-cz
-a` command with no files added to staging area, adds all files to staging and opens the prompt (no
error thrown).

"fix commitizen#785"

src/cli/strategies/git-cz.js

@@ -41,6 +41,7 @@ function gitCz (rawGitArgs, environment, adapterConfig) {
   let resolvedAdapterConfigPath = resolveAdapterPath(adapterConfig.path);
   let resolvedAdapterRootPath = findRoot(resolvedAdapterConfigPath);
   let prompter = getPrompter(adapterConfig.path);
+  let shouldStageAllFiles = rawGitArgs.includes('-a') || rawGitArgs.includes('--all');
 
   isClean(process.cwd(), function (error, stagingIsClean) {
     if (error) {
@@ -67,6 +68,6 @@ function gitCz (rawGitArgs, environment, adapterConfig) {
         throw error;
       }
     });
-  });
+  }, shouldStageAllFiles);
 
 }

src/commitizen/staging.js

@@ -5,8 +5,8 @@ export { isClean };
 /**
  * Asynchrounously determines if the staging area is clean
  */
-function isClean (repoPath, done) {
-  exec('git diff --no-ext-diff --name-only && git diff --no-ext-diff --cached --name-only', {
+function isClean (repoPath, done, stageAllFiles) {
+  exec(`git diff --cached --no-ext-diff --name-only ${!!stageAllFiles ? '&& git diff --no-ext-diff --name-only' : ''}`, {
     maxBuffer: Infinity,
     cwd: repoPath
   }, function (error, stdout) {