update links to https://github.com/git/git-scm.com/.../master

The default branch will be switching to "main". I think GitHub would
actually redirect these for us, but there's no reason not to give the
new canonical location.

Note that there are many mentions of "master" for git/git,
progit/progit2, etc. Those repositories are still using "master" for
now.

Author: peff

README.md

@@ -94,7 +94,7 @@ be accepted. If it involves code, please also write tests for it.
 
 The [list of GUI clients](https://git-scm.com/downloads/guis) has been constructed by the community for a long time. If you want to add another tool you'll need to follow a few steps:
 
-1. Add the GUI client details at the YAML file: https://github.com/git/git-scm.com/blob/master/resources/guis.yml
+1. Add the GUI client details at the YAML file: https://github.com/git/git-scm.com/blob/main/resources/guis.yml
     1. The fields `name`, `url`, `price`, `license` should be very straightforward to fill.
     2. The field `image_tag` corresponds to the filename of the image of the tool (without path, just the filename).
     3. `platforms` is a list of at least 1 platform in which the tool is supported. The possibilities are: `Windows`, `Mac`, `Linux`, `Android`, and `iOS`

app/views/downloads/guis/index.html.erb

@@ -12,7 +12,7 @@
     Git comes with built-in GUI tools for committing (<a href="/docs/git-gui">git-gui</a>) and browsing (<a href="/docs/gitk">gitk</a>), but there are several third-party tools for users looking for platform-specific experience.
   </p>
   <p>
-    <small>If you want to add another GUI tool to this list, just <a href="https://github.com/git/git-scm.com/blob/master/README.md#adding-new-gui">follow the instructions</a>.</small>
+    <small>If you want to add another GUI tool to this list, just <a href="https://github.com/git/git-scm.com/blob/main/README.md#adding-new-gui">follow the instructions</a>.</small>
   </p>
 
   <p>

app/views/site/about.html.erb

@@ -17,7 +17,7 @@
     repository at GitHub. That includes all pages except the book and
     manpages, along with the general design of the site. This content
     is available under the
-    <a href="https://github.com/git/git-scm.com/blob/master/MIT-LICENSE.txt">MIT license</a>.
+    <a href="https://github.com/git/git-scm.com/blob/main/MIT-LICENSE.txt">MIT license</a>.
     Bug reports and suggestions may be made in that repository.
 
     <li>Content from the ProGit book is imported from the
@@ -70,6 +70,6 @@
   </ul>
 
   For a detailed view of the site's network layout, see our
-  <a href="https://github.com/git/git-scm.com/blob/master/ARCHITECTURE.md">architecture document</a>.
+  <a href="https://github.com/git/git-scm.com/blob/main/ARCHITECTURE.md">architecture document</a>.
 
 </div>