gcc-descr: by default show revision for HEAD

I've made the mistake multiple times while doing a bisection.
One would expect that gcc-descr w/o an argument would print
revision for the current HEAD and not master.
Thus change it.

contrib/ChangeLog:

	* git-descr.sh: By default print revision for HEAD.

contrib/git-descr.sh

@@ -4,7 +4,7 @@
 
 short=no
 long=no
-c=master
+c=HEAD
 
 for arg in "$@"
 do