Relax the requirement for extra commands / aliases to not share prefixes

[NonLogicalDev]

Adding this to git configuration

[stgit.alias]
  show-stat = git-utils stg-showstat

Will result in:

$ stg show <patch-name>
Error: Ambiguous command: show
       Candidates are: show, show-stat

[jpgrayson]

I appreciate this issue report, @NonLogicalDev. Exact command matches are no longer treated as ambiguous.