Ensure we restrict refspec interpretation while fetching

As `git fetch` may chose to interpret refspec to it's liking, ensure that we
only pass refs that begin with `refs/` as is, otherwise, prepend them with
`refs/heads`. Otherwise, branches named `heads/foo` (I know it's bad, but it's
allowed), would be fetched as `foo`, instead of `heads/foo`.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index e069057eb..75ce5ee8b 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -282,7 +282,10 @@ struct GitInput : Input
                 // FIXME: git stderr messes up our progress indicator, so
                 // we're using --quiet for now. Should process its stderr.
                 try {
-                    runProgram("git", true, { "-C", repoDir, "fetch", "--quiet", "--force", "--", actualUrl, fmt("%s:%s", *input->ref, *input->ref) });
+                    auto fetchRef = input->ref->compare(0, 5, "refs/") == 0
+                        ? *input->ref
+                        : "refs/heads/" + *input->ref;
+                    runProgram("git", true, { "-C", repoDir, "fetch", "--quiet", "--force", "--", actualUrl, fmt("%s:%s", fetchRef, fetchRef) });
                 } catch (Error & e) {
                     if (!pathExists(localRefFile)) throw;
                     warn("could not update local clone of Git repository '%s'; continuing with the most recent version", actualUrl);