git putFile: support flake maximalists

Passing the commit message as an argument causes update failures on repositories with lots of flake inputs. In some cases, the commit message is over 250,000 bytes.

src/libfetchers/unix/git.cc

@@ -342,7 +342,8 @@ struct GitInputScheme : InputScheme
                 logger->pause();
                 Finally restoreLogger([]() { logger->resume(); });
                 runProgram("git", true,
-                    { "-C", repoInfo.url, "--git-dir", repoInfo.gitDir, "commit", std::string(path.rel()), "-m", *commitMsg });
+                    { "-C", repoInfo.url, "--git-dir", repoInfo.gitDir, "commit", std::string(path.rel()), "-F", "-" },
+                    *commitMsg);
             }
         }
     }