libfetchers: write git commit message to tempfile

we want to remove runProgram's ability to provide stdin to a process
because the concurrency issues of handling both stdin and stdout are
much more pronounced once runProgram returns not is collected output
but a source. this is possible in the current c++ framework, however
it isn't necessary in almost all cases (as demonstrated by only this
single user existing) and in much better handled with a proper async
concurrency model that lets the caller handle both at the same time.

Change-Id: I29da1e1ad898d45e2e33a7320b246d5003e7712b

src/libfetchers/git.cc

@@ -396,12 +396,15 @@ struct GitInputScheme : InputScheme
 
 
             if (commitMsg) {
+                auto [_fd, msgPath] = createTempFile("nix-msg");
+                AutoDelete const _delete{msgPath};
+                writeFile(msgPath, *commitMsg);
+
                 // Pause the logger to allow for user input (such as a gpg passphrase) in `git commit`
                 logger->pause();
                 Finally restoreLogger([]() { logger->resume(); });
                 runProgram("git", true,
-                    { "-C", *root, "--git-dir", gitDir, "commit", std::string(path.rel()), "-F", "-" },
-                    *commitMsg);
+                    { "-C", *root, "--git-dir", gitDir, "commit", std::string(path.rel()), "-F", msgPath });
             }
         }
     }