By setting the 'ident' attribute for /download_and_compile.sh, this
replacement will be automatic whenever the file is checked out.
Note: the ID is the 40-character hexadecimal blob object name in Git, it
is *not* the commit ID. 'git show <id>' can be used to obtain the
script contents from its blob object name (id).
This will avoid spoiling every diff with the version number change, or
alternatively having different versions of the same file displaying and
logging the same version number.
If download_and_compile.sh is moved to a different directory in FGMeta
(or even renamed), then the path in .gitattributes must be adapted for
the substitution to work (the '/' in .gitattributes anchors the match at
the containing directory; I did this because download_and_compile.sh is
a rather generic name, and there *might* be different scripts with the
same name in other dirs...).
