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

[Martin Pool]

Martin Pool has voted +1.
Status is now: Approved
Comment:
I had to check whether closing a file object created with fdopen closed 
the underlying file, but it does.  (I mention it just in case anyone 
else was unsure.)

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46364EF6.3040308%40ukr.net%3E