Check Git branch in production mode

When the current Git branch isn't the `main` branch, exit the `mono`
executable. This is to ensure that users don't accidentally use an
unmerged and unreviewed branch.

Developers can use the `mono-dev` executable instead.

.changesets/check-git-branch-for-mono.md

@@ -0,0 +1,5 @@
+---
+bump: "patch"
+---
+
+Check Git branch for mono to prevent accidental usage of mono on an unmerged branch.

bin/mono

@@ -16,6 +16,16 @@ unless git_status.empty?
   exit 1
 end
 
+current_branch = `git rev-parse --abbrev-ref HEAD`.chomp
+unless current_branch == "main"
+  puts "ERROR: The mono repository is not on the `main` branch. " \
+    "Please switch to the main branch to ensure you're using a released " \
+    "version."
+  puts "Please use the `mono-dev` executable if you want to use uncommitted " \
+    "changes to _test_ mono itself."
+  exit 1
+end
+
 $LOAD_PATH << File.expand_path(File.join(__dir__, "..", "lib"))
 
 require "mono"