This allows git-am to accept single-message files as well as mboxes. 
Unlike the previous version, this one doesn't need to be explicitly told 
which one it is; rather, it looks to see if the first line is a From 
line and uses it to select mbox mode or not.

I moved the logic to do all this into git-mailsplit, which got a new 
user interface as result, although the old interface is still available 
for backwards compatibility.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>