subtree: t7900: fix 'verify one file change per commit'

subtree: t7900: fix 'verify one file change per commit'

As far as I can tell, this test isn't actually testing anything, because
someone forgot to tack on `--name-only` to `git log`.  This seems to
have been the case since the test was first written, back in fa16ab36ad
("test.sh: make sure no commit changes more than one file at a time.",
2009-04-26), unless `git log` used to do that by default and didn't need
the flag back then?

Convincing myself that it's not actually testing anything was tricky,
the code is a little hard to reason about.  It can be made a lot simpler
if instead of trying to parse all of the info from a single `git log`,
we're OK calling `git log` from inside of a loop.  And it's my opinion
that tests are not the place for clever optimized code.

So, fix and simplify the test, so that it's actually testing something
and is simpler to reason about.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>