Reference bash explicitly in shebang lines

On Solaris, where /bin/sh is actually sh, not bash, some expressions
like $() are not supported.  Git's own scripts on this machine are
configured to use "#!/bin/bash".  Change our shebang line to

  #!/usr/bin/env bash

which should work almost everywhere.

commit-msg

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 #
 # Copy or link this file as ".git/hooks/pre-commit".
 

pre-commit

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 #
 # Copy or link this file as ".git/hooks/pre-commit".