Add GitDirectory.ref_exists() method

This allows checking if a ref exists without requiring the caller
to use a try:, except: block.

1 files changed, 7 insertions, 0 deletions

diff --git a/morphlib/gitdir.py b/morphlib/gitdir.py
index 9fef4f1e..fd7f3808 100644
--- a/morphlib/gitdir.py
+++ b/morphlib/gitdir.py
@@ -537,6 +537,13 @@ class GitDirectory(object):
     def resolve_ref_to_tree(self, ref):
         return self._rev_parse('%s^{tree}' % ref)
 
+    def ref_exists(self, ref):
+        try:
+            self._rev_parse('%s^{commit}' % ref)
+            return True
+        except InvalidRefError:
+            return False
+
     def _list_files_in_work_tree(self):
         for dirpath, subdirs, filenames in os.walk(self.dirname):
             if dirpath == self.dirname and '.git' in subdirs: