mergetool: style fixes

This script is one of the sizeable ones that tempted people to copy
its "neibouring style" in their new code, but was littered with
styles incompatible with our style guide.

 - use one tab, not four spaces, per indent level;

 - long lines can be wrapped after '|', '&&', or '||' for
   readability.

 - structures like "if .. then .. else .. fi", "while .. do .. done"
   are split into lines in such a way that does not require
   unnecessary semicolon.

 - case, esac and case-arms align at the same column.

Signed-off-by: Junio C Hamano <gitster@pobox.com>