download_and_compile.sh: replace manually set version with the Git blob ID

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...).

.gitattributes

@@ -20,3 +20,4 @@ README.*                       text
 Makefile                       text
 ChangeLog                      text
 ChangeLog.*                    text
+/download_and_compile.sh       text ident

download_and_compile.sh

@@ -17,7 +17,15 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-VERSION="2.34"
+script_blob_id='$Id$'
+# Slightly tricky substitution to avoid our regexp being wildly replaced with
+# the blob name (id) when the script is checked out:
+#
+# First extract the hexadecimal blob object name followed by a '$'
+VERSION="$(echo "$script_blob_id" | sed 's@\$Id: *\([0-9a-f]\+\) *@\1@')"
+# Then remove the trailing '$'
+VERSION="${VERSION%\$}"
+
 FGVERSION="release/$(git ls-remote --heads https://git.code.sf.net/p/flightgear/flightgear|grep '\/release\/'|cut -f4 -d'/'|sort -t . -k 1,1n -k2,2n -k3,3n|tail -1)"
 
 #######################################################