Use one message for both pushes on hotfix merge.

The user will be prompted to enter a message in a tmpfile which will
then be concatenated to both merge descriptions.

This is to prevent having to write the same message twice.
Use read instead of each_line.

I don't need to process each line of the file so it would be more
efficient to just read the file.
Remove typecasting.

'filename' is already a string so it's silly to attempt to typecast a
string into a string.
Modify editor error message.

Use a colon to implicitly suggest what you need to do. Add a newline for
clarity.
Move require within the function.
Use git-var instead of git-config.
Make editor params more flexible.

The lack of duplication will allow additional editors to be added more
easily.
Use tempfile module instead of securerandom.
Remove unneeded comment.
Prompt user only once for their commit message.

By using no-edit, git merge will automatically accept the message called
with it and no longer call $EDITOR.

I also shellescaped hotfix instead of using quotes. I feel that calling
this function results in the code being more explicit, because the coder
will easily recognize that the string is being shellescaped (rather than
relying on their knowledge of the effect of double quotes in bash).
Prompt user to edit each different commit description.
Ues unless instea of if !(expression).

It's faster.
Remove unneeded comments. Adjust punctuation/grammar.
Use only one call to API for pull description.

Now, the pull description for the dev_branch will be formulated by
subbing the first instance of 'stable' with dev_branch. This also
alleviates the issue of having to open an $EDITOR twice.
Merge add-commit-prompt-for-hotfix-merge (#78) into master

Use one message for both pushes on hotfix merge.

The user will be prompted to enter a message in a tmpfile which will
then be concatenated to both merge descriptions.

This is to prevent having to write the same message twice.

Fixes #19.