Minor builtin 'git am' side-effect

Hi,

The format of the files '.git/rebase-apply/{next,last}' changed slightly
with the recent builtin 'git am' conversion: while these files were
newline-terminated when written by the scripted version, the ones written
by the builtin are not.

This probably  makes no difference for shell scripts looking at these
files, e.g.  our __git_ps1() handles both just fine.  However, it can
break C programs, when after strtol()ing the contents of the files they
get defensive and check for the terminating newline at *endptr (this is
how I noticed, as one of my pet projects did just that).

I'm not saying that the new behavior is bad and should be fixed; I merely
point it out and leave the rest for you to decide.

Best,
Gábor