R1:5ffdb732735c

Don't try to prune unreachable commits from repositories with no outdated refs

Summary:
Fixes T11269. The basic issue is that `git log` in an empty repository exits with an error message.

Prior to recent Git (2.6?), this message reads:

> fatal: bad default revision 'HEAD'

This message was somewhat recently changed by <https://github.com/git/git/commit/ce113604672fed9b429b1c162b1005794fff6a17>. After that, it reads:

> fatal: your current branch 'master' does not have any commits yet

This change isn't…