fetch_git_sha for Sentry

_README.md

Fetch the git SHA, with packed-refs support, and fallback to HEAD based on https://github.com/getsentry/raven-python/blob/25728c8190a7f008cdb00b1664cd4fd479060eaf/raven/versioning.py#L17

fetch_git_sha.py

def fetch_git_sha(path):
    head_path = os.path.join(path, '.git', 'HEAD')
    if not os.path.exists(head_path):
        return ''

    with open(head_path) as f:
        head = f.read().strip()
    head = head[5:]

    revision_file = os.path.join(path, '.git', *head.split('/'))
    if not os.path.exists(revision_file):
        # Check for our .git/packed-refs' file since a `git gc` may have run
        # https://git-scm.com/book/en/v2/Git-Internals-Maintenance-and-Data-Recovery
        packed_file = os.path.join(path, '.git', 'packed-refs')
        if os.path.exists(packed_file):
            with open(packed_file, 'r') as f:
                 for line in f:
                     line = line.rstrip()
                     if not line:
                         continue
                     if line[:1] in ('#', '^'):
                         continue
                     try:
                         revision, ref = line.split(' ', 1)
                     except ValueError:
                         continue
                     if ref == head:
                         return revision
    else:
        with open(revision_file) as f:
            return f.read().strip()

    # default to HEAD
    return head