Merge pull request #977 from betatim/support-main-and-master

[MRG] Remove reference to `master` branch from CLI doc

repo2docker/__main__.py

@@ -96,7 +96,8 @@ def get_argparser():
     argparser.add_argument(
         "--ref",
         help=(
-            "If building a git url, which reference to check out. " "E.g., `master`."
+            "Reference to build instead of default reference. For example"
+            " branch name or commit for a Git repository."
         ),
     )
 

repo2docker/contentproviders/git.py

@@ -47,9 +47,18 @@ class Git(ContentProvider):
                 self.log.error(
                     "Failed to check out ref %s", ref, extra=dict(phase="failed")
                 )
-                raise ValueError("Failed to check out ref {}".format(ref))
+                if ref == "master":
+                    msg = (
+                        "Failed to check out the 'master' branch. "
+                        "Maybe the default branch is not named 'master' "
+                        "for this repository.\n\nTry not explicitly "
+                        "specifying `--ref`."
+                    )
+                else:
+                    msg = "Failed to check out ref {}".format(ref)
+                raise ValueError(msg)
             # We don't need to explicitly checkout things as the reset will
-            # take of that. If the hash is resolved above, we should be
+            # take care of that. If the hash is resolved above, we should be
             # able to reset to it
             for line in execute_cmd(
                 ["git", "reset", "--hard", hash], cwd=output_dir, capture=yield_output