git: add osxkeychainSupport option (#112503)

in order to be able to disable osxkeychain credential helper if no
wanted / necessary. Keep it `true` by default such that current logic is
not affected.

Signed-off-by: Andreas Schmid <[e-mail]>

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

···
 , svnSupport, subversionClient, perlLibs, smtpPerlLibs
 , perlSupport ? true
 , nlsSupport ? true
+, osxkeychainSupport ? stdenv.isDarwin
 , guiSupport
 , withManual ? true
 , pythonSupport ? true
···
 , gzip # needed at runtime by gitweb.cgi
 }:
 
+assert osxkeychainSupport -> stdenv.isDarwin;
 assert sendEmailSupport -> perlSupport;
 assert svnSupport -> perlSupport;
 
···
     make -C contrib/subtree
   '' + (lib.optionalString perlSupport ''
     make -C contrib/diff-highlight
-  '') + (lib.optionalString stdenv.isDarwin ''
+  '') + (lib.optionalString osxkeychainSupport ''
     make -C contrib/credential/osxkeychain
   '') + (lib.optionalString withLibsecret ''
     make -C contrib/credential/libsecret
···
 
   installFlags = [ "NO_INSTALL_HARDLINKS=1" ];
 
-  preInstall = (lib.optionalString stdenv.isDarwin ''
+  preInstall = (lib.optionalString osxkeychainSupport ''
     mkdir -p $out/bin
     ln -s $out/share/git/contrib/credential/osxkeychain/git-credential-osxkeychain $out/bin/
     rm -f $PWD/contrib/credential/osxkeychain/git-credential-osxkeychain.o
···
          notSupported "$out/$prog"
        done
      '')
-   + lib.optionalString stdenv.isDarwin ''
-    # enable git-credential-osxkeychain by default if darwin
+   + lib.optionalString osxkeychainSupport ''
+    # enable git-credential-osxkeychain on darwin if desired (default)
     mkdir -p $out/etc
     cat > $out/etc/gitconfig << EOF
     [credential]