change update-revision to use #!/bin/sh

Previously it was using #!/bin/bash as its interpreter. It is not
compatible with FreeBSD because bash is not installed by default.

The update-revision script is #!/bin/sh compatible and there are
other examples of #!/bin/sh scripts in the codebase so the change
is consistent with them.

original commit: 0ad188cdbd618b369e7fe6d2b9446251fe92e891

s/update-revision

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 if [ -d ../../.git ]; then
   git describe --always --exclude='*' --abbrev=40 --dirty
   echo 'git'