Ensure that a repository's path (at initialization or open time) is
valid.  On Windows systems, this means that the longest known path
beneath the repository will fit within MAX_PATH: this is a lock file for
a loose object within the repository itself.

Other paths, like a very long loose reference, may fail to be opened
after the repository is opened.  These variable length paths will be
checked when they are accessed themselves.  This new functionality is
done at open to prevent needlessly checking every file in the gitdir
(eg, `MERGE_HEAD`) for its length when we could instead check once at
repository open time.