gitsocial memo show 4 matches two project memos, 4a23276c29e2 and
4d2b160c7d7f, and returns one without an error.

Reproduce with gitsocial memo list, then gitsocial memo show on a
prefix two hashes share.

CLI.md documents a short hash prefix as working when unambiguous.

Expected: an error listing every matching commit, as git show does
for the same prefix.