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.

Upstream PR: https://github.com/NixOS/nix/pull/10686

(cherry picked from commit 8b5e8f4fba5728f2b3e90fcd1ab15df77e3ea0e8)
Change-Id: I2c196a21cc9bedc24d57a828a0c5b9467e072f76

src/libfetchers/git.cc

@@ -402,7 +402,8 @@ struct GitInputScheme : InputScheme
                 logger->pause();
                 Finally restoreLogger([]() { logger->resume(); });
                 runProgram("git", true,
-                    { "-C", *root, "--git-dir", gitDir, "commit", std::string(path.rel()), "-m", *commitMsg });
+                    { "-C", *root, "--git-dir", gitDir, "commit", std::string(path.rel()), "-F", "-" },
+                    *commitMsg);
             }
         }
     }