Use cwd switching to do git operations

Setting just --git-dir is not enough, you also need to set
--work-dir. Just setting cwd is better

builder/app.py

@@ -83,7 +83,7 @@ class Builder(Application):
             sys.exit(1)
 
         try:
-            for line in execute_cmd(['git', '--git-dir', os.path.join(output_path, '.git'), 'reset', '--hard', ref]):
+            for line in execute_cmd(['git', 'reset', '--hard', ref], output_path):
                 self.log.info(line, extra=dict(phase='fetching'))
         except subprocess.CalledProcessError:
             self.log.error('Failed to check out ref %s', ref, extra=dict(phase='failed'))

builder/detectors.py

@@ -65,12 +65,12 @@ class S2IBuildPack(BuildPack):
             'build',
             '--exclude', '""',
             '--ref', ref,
-            workdir,
+            '.',
             build_image,
             output_image_spec,
         ]
         try:
-            for line in execute_cmd(cmd):
+            for line in execute_cmd(cmd, cwd=workdir):
                 self.log.info(line, extra=dict(phase='building', builder=self.name))
         except subprocess.CalledProcessError:
             self.log.error('Failed to build image!', extra=dict(phase='failed'))

builder/utils.py

@@ -1,10 +1,10 @@
 import subprocess
 
-def execute_cmd(cmd):
+def execute_cmd(cmd, cwd=None):
     """
     Call given command, yielding output line by line
     """
-    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, universal_newlines=True)
+    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, universal_newlines=True, cwd=cwd)
 
     try:
         for line in iter(proc.stdout.readline, ''):