Currently, the process for getting the common prefix of a list of file
names is a tad too greedy:
    1. Find the common prefix of all the strings.
    2. Strip the extension from the prefix.
    3. Strip trailing punctuation.

Step 2 may strip dots if there’s trailing whitespace and step 3 may
strip useful punctuation (e.g. parentheses). This commit reworks the
process as such:
    1. Strip the extension from all the file names.
    2. Find the common prefix of all the strings.
    3. Trim trailing whitespace.

Fixes #174.