checkout: factor out functions to new lib file

Factor the functions out, so they can be re-used from other places.  In
particular these functions will be re-used in builtin/worktree.c to make
git worktree add dwim more.

While there add some docs to the function.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

checkout.h

@@ -0,0 +1,13 @@
+#ifndef CHECKOUT_H
+#define CHECKOUT_H
+
+#include "cache.h"
+
+/*
+ * Check if the branch name uniquely matches a branch name on a remote
+ * tracking branch.  Return the name of the remote if such a branch
+ * exists, NULL otherwise.
+ */
+extern const char *unique_tracking_name(const char *name, struct object_id *oid);
+
+#endif /* CHECKOUT_H */