Support fetching from an upstream main git branchandroid-s-beta-2android-s-beta-1

The external updater currently assumes the name of the upstream git
branch to fetch is always "master".  This is no longer true, as some
repositories use "main" instead.  This commit lets the updater try to
use both of those branches.

Note that there are still a few instances of "master" in the updater,
but I believe those correspond to AOSP and GitHub specifically and so
shouldn't need to be changed yet.  If/when they do, we should be able
to change them easily with these changes.

As part of this change we also fix a potential bug in a
previously-unused function so that it does not strip part of the
branch name from a string of the form "<remote name>/<branch name>".

Test: Run the updater with upstream master and upstream main.
Change-Id: I0cf83dbf1765ff26b602176fa1c1ff95224e1a7f

2 files changed, 16 insertions, 2 deletions

diff --git a/git_updater.py b/git_updater.py
index 4c02821..ef320cb 100644
--- a/git_updater.py
+++ b/git_updater.py
@@ -66,8 +66,10 @@ class GitUpdater(base_updater.Updater):
         self._new_ver = updater_utils.get_latest_version(self._old_ver, tags)
 
     def _check_head(self) -> None:
+        branch = git_utils.get_default_branch(self._proj_path,
+                                              self.UPSTREAM_REMOTE_NAME)
         self._new_ver = git_utils.get_sha_for_branch(
-            self._proj_path, self.UPSTREAM_REMOTE_NAME + '/master')
+            self._proj_path, self.UPSTREAM_REMOTE_NAME + '/' + branch)
 
     def update(self) -> None:
         """Updates the package.
diff --git a/git_utils.py b/git_utils.py
index 1825c37..f96a600 100644
--- a/git_utils.py
+++ b/git_utils.py
@@ -99,7 +99,7 @@ def list_remote_branches(proj_path: Path, remote_name: str) -> List[str]:
     stripped = [line.strip() for line in lines]
     remote_path = remote_name + '/'
     return [
-        line.lstrip(remote_path) for line in stripped
+        line[len(remote_path):] for line in stripped
         if line.startswith(remote_path)
     ]
 
@@ -116,6 +116,18 @@ def list_remote_tags(proj_path: Path, remote_name: str) -> List[str]:
     return list(set(tags))
 
 
+def get_default_branch(proj_path: Path, remote_name: str) -> str:
+    """Gets the name of the upstream branch to use."""
+    branches_to_try = ['master', 'main']
+    remote_branches = list_remote_branches(proj_path, remote_name)
+    for branch in branches_to_try:
+        if branch in remote_branches:
+            return branch
+    # We couldn't find any of the branches we expected.
+    # Default to 'master', although nothing will work well.
+    return 'master'
+
+
 COMMIT_PATTERN = r'^[a-f0-9]{40}$'
 COMMIT_RE = re.compile(COMMIT_PATTERN)