Extract git reference parsing to a shared library

These utility functions can be shared between the git and github fetchers.

src/libfetchers/git-utils.hh

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <string>
+#include <string_view>
+#include <optional>
+
+// Parses the HEAD ref as reported by `git ls-remote --symref`
+//
+// Returns the head branch name as reported by `git ls-remote --symref`, e.g., if
+// ls-remote returns the output below, "main" is returned based on the ref line.
+//
+//   ref: refs/heads/main       HEAD
+//
+// If the repository is in 'detached head' state (HEAD is pointing to a rev
+// instead of a branch), parseListReferenceForRev("HEAD") may be used instead.
+std::optional<std::string> parseListReferenceHeadRef(std::string_view line);
+
+// Parses a reference line from `git ls-remote --symref`, e.g.,
+// parseListReferenceForRev("refs/heads/master", line) will return 6926...
+// given the line below.
+//
+// 6926beab444c33fb57b21819b6642d032016bb1e	refs/heads/master
+std::optional<std::string> parseListReferenceForRev(std::string_view rev, std::string_view line);