mergetool: move main program flow into a main() function

mergetool: move main program flow into a main() function

Make it easier to follow the program's flow by isolating all
logic into functions.  Isolate the main execution code path into
a single unit instead of having prompt_after_failed_merge()
interrupt it partyway through.

The use of a main() function is borrowing a convention from C,
Python, Perl, and many other languages.  This helps readers more
familiar with other languages understand the purpose of each
function when diving into the codebase with fresh eyes.

Signed-off-by: David Aguilar <davvid@gmail.com>
Reviewed-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>