Adjust to only show merges into master

Adding `--first-parent` ensures that only commits from the master branch
are used when showing the history. I've verified this change directly
by comparing the `rev-list` output for `703105c..97df5c45`:

```console
$ git rev-list --merges 703105c...97df5c4
97df5c4540
b5976ada25
$ git rev-list --merges --first-parent 703105c...97df5c4
97df5c4540
```

Note: I've created this branch from 703105c so that I can merge `master`
into it and test `bash-it update` once merged.

lib/helpers.bash

@@ -182,7 +182,7 @@ _bash-it_update() {
 
   if [[ -n "${status}" ]]; then
 
-    for i in $(git rev-list --merges master..${BASH_IT_REMOTE}); do
+    for i in $(git rev-list --merges --first-parent master..${BASH_IT_REMOTE}); do
       num_of_lines=$(git log -1 --format=%B $i | awk 'NF' | wc -l)
       if [ $num_of_lines -eq 1 ]; then
         description="%s"