mergetool: use shell variable magic instead of `awk`

mergetool: use shell variable magic instead of `awk`

git-mergetool spawns an enormous amount of processes. For this reason,
the test script, t7610, is exceptionally slow, in particular, on
Windows. Most of the processes are invocations of git. There are
also some that can be replaced with shell builtins. Avoid repeated
calls of `git ls-files` and `awk`.

Signed-off-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>