Fix gitignores

[lutostag]

Found that some of the exercises had different gitignores. Caused me to almost commit the /target/ files to my exercises when running through them (reverse string was the first I ran into with the problem).

Made sure they all have the same now:

# Generated by Cargo
# will have compiled files and executables
/target/
**/*.rs.bk

# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
# More information here http://doc.crates.io/guide.html#cargotoml-vs-cargolock
Cargo.lock

Also updated the init_exercise.py to have the same gitignore for future created projects.

You can check they are all the same (except for grep) which I also included the *.txt because it seemed like some of the tests may have large text files there... by running
md5sum exercises/*/.gitignore | sort

Hope this is helpful.

Thanks!

[coriolinus]

Thanks, @lutostag, for doing this work!

In general, we prefer to keep the scope of each PR separate. It's better to have one PR which updates the necessary readme files, and another which must be rebased after the first is merged, than to have multiple unrelated changes in a single PR. See also here for another track maintainer's perspective.

As such, please remove the changes to non-.gitignore files from this PR. If this causes Travis to fail, then so be it; Travis is advisory, not mandatory, when choosing whether or not to merge.

[coriolinus]

This has already been updated by #597. Please remove this change from this PR.

[lutostag]

@coriolinus Thanks for the helpful pointers, I should have checked the PRs first -- oops...

I removed the extraneous changes, cleaned history up and rebased on latest master.
(I tried to follow the commit guidelines as well -- missed that first time around).

Let me know if anything else needs fixing. Thanks!

[coriolinus]

Thanks! There's one more change; you overwrote a .gitignore which had already been customized. After this, I think we'll be all set.

[coriolinus]

This line should not be removed; the grep exercise generates some text files, and while it is meant to clean up after itself, it might not in the event of panics.

[lutostag]

It's moved to the top line. Still there. I'll move it back to the bottom

[lutostag]

Using your explanation as a comment there too. Thanks!

[coriolinus]

Thanks for doing this, @lutostag! Next steps: I'm going to leave this open for a while to give the other track maintainers the option to take a look at it and make comments as desired.

Assuming there are no objections, I intend to merge this not later than Wednesday the 8th. If on Thursday I haven't yet done so, please feel free to ping me a reminder.