Use lib.pathIsGitRepo

instead of just checking for the existence of `.git`. It covers more
cases that way.

Author: Shay Bergmann

nix/pkgs/mantis/default.nix

@@ -2,15 +2,16 @@
 
   mkSrc = src:
     let
-      isGit = builtins.pathExists (src + "/.git");
+      isGit = lib.pathIsGitRepo (src + "/.git");
       repo = builtins.fetchGit { url = src; submodules = true; };
       dirty = repo.revCount == 0;
       filterSrc = src:
         lib.cleanSourceWith {
           inherit src;
           filter = path: _: !lib.hasSuffix "nix" path;
         };
-    in if isGit then
+    in
+    if isGit then
       if dirty then filterSrc (gitignoreSource src) else repo
     else
       src;