add -i (built-in): do show an error message for incorrect inputs

add -i (built-in): do show an error message for incorrect inputs

There is a neat feature in `git add -i` where it allows users to select
items via unique prefixes.

In the built-in version of `git add -i`, we specifically sort the items
(unless they are already sorted) and then perform a binary search to
figure out whether the input constitutes a unique prefix. Unfortunately,
by mistake this code misidentifies matches even if the input string is
not actually a prefix of any item.

For example, in the initial menu, where there is a `status` and an
`update` command, the input `tadaa` was mistaken as a prefix of
`update`.

Let's fix this by looking a bit closer whether the input is actually a
prefix of the item at the found insert index.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>