(minor) bash -> sh changes in some non-core code

/bin/bash is muscle memory for me, although it appears that not too much
of the actual code is bash-specific, so it's reasonably easy to fix.

src/triggers/post-compile/update-description-file

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 # For normal (not "wild") repos, gitolite v3 sets 'gitweb.description' instead
 # of putting the text in the "description" file.  This is easier because it