Generalize merge conflict message

Author: Kobzol

README.md

@@ -15,12 +15,12 @@ integration service such as [Travis CI], [Appveyor] or [Buildbot].
 
 Let's take Travis CI as an example. If you send a pull request to a repository,
 Travis CI instantly shows you the test result, which is great. However, after
-several other pull requests are merged into the `master` branch, your pull
-request can *still* break things after being merged into `master`. The
+several other pull requests are merged into the default branch, your pull
+request can *still* break things after being merged into the default branch. The
 traditional continuous integration solutions don't protect you from this.
 
 In fact, that's why they provide the build status badges. If anything pushed to
-`master` is completely free from any breakage, those badges will **not** be
+the default branch is completely free from any breakage, those badges will **not** be
 necessary, as they will always be green. The badges themselves prove that there
 can still be some breakages, even when continuous integration services are used.
 
@@ -30,7 +30,7 @@ merge*, not just after the pull request is received. You can manually click the
 automate this process. It listens to the pull request comments, waiting for an
 approval comment from one of the configured reviewers. When the pull request is
 approved, Homu tests it using your favorite continuous integration service, and
-only when it passes all the tests, it is merged into `master`.
+only when it passes all the tests, it is merged into the default branch.
 
 Note that Homu is **not** a replacement of Travis CI, Buildbot or Appveyor. It
 works on top of them. Homu itself doesn't have the ability to test pull

homu/main.py

@@ -938,16 +938,17 @@ def create_merge(state, repo_cfg, branch, logger, git_cfg,
 
     desc = 'Merge conflict'
     comment = (
-        'This pull request and the master branch diverged in a way that cannot'
-        ' be automatically merged. Please rebase on top of the latest master'
+        'This pull request and the default branch diverged in '
+        'a way that cannot'
+        ' be automatically merged. Please rebase on top of the latest default'
         ' branch, and let the reviewer approve again.\n'
         '\n'
         '<details><summary>How do I rebase?</summary>\n\n'
         'Assuming `self` is your fork and `upstream` is this repository,'
         ' you can resolve the conflict following these steps:\n\n'
         '1. `git checkout {branch}` *(switch to your branch)*\n'
-        '2. `git fetch upstream master` *(retrieve the latest master)*\n'
-        '3. `git rebase upstream/master -p` *(rebase on top of it)*\n'
+        '2. `git fetch upstream HEAD` *(retrieve the latest HEAD)*\n'
+        '3. `git rebase upstream/HEAD -p` *(rebase on top of it)*\n'
         '4. Follow the on-screen instruction to resolve conflicts'
         ' (check `git status` if you got lost).\n'
         '5. `git push self {branch} --force-with-lease` *(update this PR)*\n\n'
@@ -1335,7 +1336,7 @@ def start_build(state, repo_cfgs, buildbot_slots, logger, db, git_cfg):
                     continue
                 builders += ['status-' + key]
                 # We have an optional fast path if the Travis test passed
-                # for a given commit and master is unchanged, we can do
+                # for a given commit and main is unchanged, we can do
                 # a direct push.
                 if context == 'continuous-integration/travis-ci/push':
                     found_travis_context = True