cmd/update: attempt migration from master to main branch.

This should allow any repositories that change from `master` to `main`
to be automatically migrated by `brew update` without requiring manual
intervention or displaying errors to the user.

Library/Homebrew/cmd/update.sh

@@ -725,17 +725,40 @@ EOS
       local tmp_failure_file="${DIR}/.git/TMP_FETCH_FAILURES"
       rm -f "${tmp_failure_file}"
 
-      if [[ -n "${HOMEBREW_UPDATE_AUTO}" ]]
-      then
-        git fetch --tags --force "${QUIET_ARGS[@]}" origin \
-          "refs/heads/${UPSTREAM_BRANCH_DIR}:refs/remotes/origin/${UPSTREAM_BRANCH_DIR}" 2>/dev/null
-      else
-        # Capture stderr to tmp_failure_file
       if ! git fetch --tags --force "${QUIET_ARGS[@]}" origin \
          "refs/heads/${UPSTREAM_BRANCH_DIR}:refs/remotes/origin/${UPSTREAM_BRANCH_DIR}" 2>>"${tmp_failure_file}"
       then
+        if [[ -f "${tmp_failure_file}" ]]
+        then
+          local git_errors
+          git_errors="$(cat "${tmp_failure_file}")"
+          if [[ "${git_errors}" == "fatal: couldn't find remote ref refs/heads/master" ]]
+          then
+            # Attempt migration from master to main branch.
+            if git fetch --tags --force "${QUIET_ARGS[@]}" origin \
+               "refs/heads/main:refs/remotes/origin/main" 2>>"${tmp_failure_file}"
+            then
+              rm -f "${DIR}/.git/refs/remotes/origin/HEAD" "${DIR}/.git/refs/remotes/origin/master"
+              UPSTREAM_BRANCH_DIR="$(upstream_branch)"
+              declare UPSTREAM_BRANCH"${TAP_VAR}"="${UPSTREAM_BRANCH_DIR}"
+              git branch -m master main "${QUIET_ARGS[@]}"
+              git branch -u origin/main main "${QUIET_ARGS[@]}"
+              rm -f "${tmp_failure_file}"
+              exit
+            fi
+          fi
+
+          rm -f "${tmp_failure_file}"
+        fi
+
+        # Don't output errors if HOMEBREW_UPDATE_AUTO is set.
+        if [[ -n "${HOMEBREW_UPDATE_AUTO}" ]]
+        then
+          exit
+        fi
+
         # Reprint fetch errors to stderr
-          [[ -f "${tmp_failure_file}" ]] && cat "${tmp_failure_file}" 1>&2
+        [[ -n "${git_errors}" ]] && echo "${git_errors}" 1>&2
 
         if [[ "${UPSTREAM_SHA_HTTP_CODE}" == "404" ]]
         then
@@ -751,7 +774,6 @@ EOS
           fi
         fi
       fi
-      fi
 
       rm -f "${tmp_failure_file}"
     ) &