builtin-merge: allow using a custom strategy

builtin-merge: allow using a custom strategy

Allow using a custom strategy, as long as it's named git-merge-foo. The
error handling is now done using is_git_command(). The list of available
strategies is now shown by list_commands().

If an invalid strategy is supplied, like -s foobar, then git-merge would
list all git-merge-* commands. This is not perfect, since for example
git-merge-index is not a valid strategy.

These are removed from the output by scanning the list of main commands;
if the git-merge-foo command is listed in the all_strategy list, then
it's shown, otherwise excluded. This does not exclude commands somewhere
else in the PATH, where custom strategies are expected.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>