Bug: commit -p breaks with -m

With git 1.8.3.3,

 $ seq 5 >data
 $ git add data
 $ git commit -mdata
 $ sed -i '2 d' data
 $ git commit -pmchange

At the prompt, type e to edit the hunk. The editor doesn't start, but git records a commit.

I found that builtin/commit.c sets the GIT_EDITOR env var to ":" when the user specifies the -m option. This was done in 406400ce4f69. Removing these two lines,

 if (!use_editor)
     setenv("GIT_EDITOR", ":", 1);

seems to fix the issue, but I'm not sure this won't break the prepare-commit-msg hook. I'd like to submit a patch: can I get a hint if this change would break commit hooks or anything else I'm not seeing?