git: ensure we are using the correct shell binary for tests

Previosuly the test scripts used /bin/sh which is a bit of an impurity.
It is mostly well-behaved but it essentially leaks the hosts state into
the build as /bin/sh points to some minimal shell implementation
configured on the host OS.

By patching the shebgangs of all the test scripts in the test folder
(t/*.sh) we can make sure that those run with the correct shell binary.

This was found as part of a random build failure of gitMinimal in
response to the systemd v249 PR being merged [0]. Since we have to
somehow touch the hash of the derivation to make the build failure go
away we might as well fix the hardcoded /bin/sh issue.

[0] 64556974b6 (commitcomment-56385360)

pkgs/applications/version-management/git-and-tools/git/default.nix

@@ -65,6 +65,9 @@ stdenv.mkDerivation {
     # Fix references to gettext introduced by ./git-sh-i18n.patch
     substituteInPlace git-sh-i18n.sh \
         --subst-var-by gettext ${gettext}
+
+    # ensure we are using the correct shell when executing the test scripts
+    patchShebangs t/*.sh
   '';
 
   nativeBuildInputs = [ gettext perlPackages.perl makeWrapper ]