Fix git_commit_dates() to handle (obvious) duplicates

Since git show will suppress duplicate results, change
git_commit_dates() to detect that situation and repair the damage.

Though currently, it can only handle cases where the duplicates are
obvious, i.e. have the same exact string in the argument list.  It
won't be able to handle cases where two arguments differ, but resolve
to the same underlying hash.  For our current purposes, that should be
fine since we only pass in hashes.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>