avocado/core/job.py: don't attempt to run git if .git doesn't exist

When looking for the GIT commit info to log in the Avocado job log,
we can optmize things a little by not running git if the ".git" directory
doesn't exist at the top level.

py2to3 is not being used anymore, so we don't need to check if we're
running from modules from the build directory.

Also, as another optmization, let's not look for the git binary until
we're sure we'll be running it.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/job.py

@@ -329,23 +329,28 @@ class Job(object):
 
     @staticmethod
     def _get_avocado_git_version():
+        # if running from git sources, there will be a ".git" directory
+        # 3 levels up
+        base_dir = os.path.dirname(os.path.dirname(os.path.dirname(__file__)))
+        git_dir = os.path.join(base_dir, '.git')
+        if not os.path.isdir(git_dir):
+            return
+        if not os.path.exists(os.path.join(base_dir, 'python-avocado.spec')):
+            return
+
         try:
             git = path.find_command('git')
         except path.CmdNotFoundError:
             return
-        # We need to get git root as `py2to3` creates this file in BUILD
+
         olddir = os.getcwd()
         try:
-            os.chdir(os.path.dirname(__file__))
-            git_root = process.run('%s rev-parse --show-toplevel' % git,
-                                   ignore_status=True, verbose=False)
-            if git_root.exit_status == 0 and os.path.exists(os.path.join(
-                    git_root.stdout.strip(), 'python-avocado.spec')):
-                cmd = "%s show --summary --pretty='%%H'" % git
-                res = process.run(cmd, ignore_status=True, verbose=False)
-                if res.exit_status == 0:
-                    top_commit = res.stdout.splitlines()[0][:8]
-                    return " (GIT commit %s)" % top_commit
+            os.chdir(base_dir)
+            cmd = "%s show --summary --pretty='%%H'" % git
+            res = process.run(cmd, ignore_status=True, verbose=False)
+            if res.exit_status == 0:
+                top_commit = res.stdout.splitlines()[0][:8]
+                return " (GIT commit %s)" % top_commit
         finally:
             os.chdir(olddir)