tig show with partial SHAs

[rfs613]

Not sure if this is an intended feature or a bug...

The behaviour of tig show, when given a partial SHA on the command line, appears to be:

• if the SHA is long enough to be unique, then operate on that SHA
• if there is no match for the given (partial) sha, then operate on HEAD
• if there are multiple matches, then operate on HEAD

While the first case is correct, the latter two cases are unexpected. The behaviour differs from plain git show where the latter two cases result in error message being displayed. Showing HEAD commit when it was not requested seems odd, and potentially very confusing to user.

I see this with both tig 2.3 (Ubuntu 18.04) and 2.5 (Fedora 31).