teach mklog to look in the current directory for ChangeLog files
when run in repos other than gcc mklog fails to find ChangeLog files
because it looks for $0/../$dir/ChangeLog, but of course if the diff is
for a project other than gcc that might not exist. It should be fine to
also look for $cwd/$dir/ChangeLog, and use that if we find it. This
means that for example in binutils-gdb.git you can do git commit,
and then in your editor read git diff HEAD~ | mklog - to generate a
template ChangeLog for that commit.
contrib/ChangeLog:
2016-03-06 Trevor Saunders <tbsaunde+gcc@tbsaunde.org>
* mklog: Look for the ChangeLog file in $cwd.
From-SVN: r234014