lib: fix commitIdFromGitRepo (#117752)

When in the presence of worktrees, it happens that /commondir has a
trailing slash.

In these circumstances, it can lead to `lib.pathType` being passed paths
like `/foo/bar/.git/`, which in turn lead to
`error: attribute '.git' missing`.

With this change, we now make sure send properly-formatted paths to all
other functions.

This, in particular, fixes running NixOS tests on worktrees created by
libgit2 on my machine. (Worktrees created by git itself appear to not
hit the issue.)

1 files changed, 7 insertions, 6 deletions

diff --git a/lib/sources.nix b/lib/sources.nix
index 1a3afcae67d..1a821f55056 100644
--- a/lib/sources.nix
+++ b/lib/sources.nix
@@ -138,12 +138,13 @@ rec {
              in if m == null
                 then throw ("File contains no gitdir reference: " + path)
                 else
-                  let gitDir     = absolutePath (dirOf path) (lib.head m);
-                      commonDir' = if pathIsRegularFile "${gitDir}/commondir"
-                                   then lib.fileContents "${gitDir}/commondir"
-                                   else gitDir;
-                      commonDir  = absolutePath gitDir commonDir';
-                      refFile    = lib.removePrefix "${commonDir}/" "${gitDir}/${file}";
+                  let gitDir      = absolutePath (dirOf path) (lib.head m);
+                      commonDir'' = if pathIsRegularFile "${gitDir}/commondir"
+                                    then lib.fileContents "${gitDir}/commondir"
+                                    else gitDir;
+                      commonDir'  = lib.removeSuffix "/" commonDir'';
+                      commonDir   = absolutePath gitDir commonDir';
+                      refFile     = lib.removePrefix "${commonDir}/" "${gitDir}/${file}";
                   in readCommitFromFile refFile commonDir
 
            else if pathIsRegularFile fileName