Previously, if a branch name had slashes in it, the `drop` command would
fail. This is because it used `sed` with `/` separators, and the
unescaped slashes in the branch name would mess up sed's pattern.

Change this to use grep instead of sed so we don't have to worry about
escaping and special characters in branch names.

It wasn't possible to save branches with similar names before:

    $ git branch-list save a-branch-with-suffix
    $ git branch-list save a-branch  #=> Errors
    $ git branch-list
      1 a-branch-with-suffix

This is because I wasn't matching against the whole line, so partial
matches would set off the condition and exit. Use grep's `--line-regexp`
flag to match against whole lines.

Thanks to John Kugelman for the flag:
https://stackoverflow.com/questions/4709912/how-to-make-grep-only-match-if-the-entire-line-matches/4709925#4709925

Previously if you had multiple branches with similar names, they could
all be deleted when deleting a single one.

For example:

    $ git branch-list
      1 a-branch-with-suffix
      2 a-branch
    $ git branch-list drop 2
    $ git branch-list
    $

Change the regex used to match branches to drop to match the whole line,
so we don't accidentally delete a branch when it has a positive partial
match.

Describe new features since the initial release.