(The canonical list of mergetools is in the file git-mergetool--lib, but
another copy of the list appears in the bash-completion file, and it
needs fixing there too.)

git-mergetool has a list of merge tools that it uses in order of
preference.  The order depends on things like desktop environment and
$EDITOR, but among others, it includes "bc".  This was added upstream in
2.2.0 to refer to the tool "Beyond Compare".  However, on UNIX and Linux
systems, the tool named "bc" refers to the command-line calculator in
the package of the same name.  git-mergetool should not scan for and use
a binary named "bc".

- Josh Triplett