[MERGE][0.16] temp file for commit message in external editor should be opened in text mode (#110901)

[Robert Collins]

Robert Collins has voted +1.
Status is now: Approved
Comment:
Merging this to 0.16 and integration. Lets keep the bug open until we 
have a test though?

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46344A73.1050002%40ukr.net%3E