publish: Ensure pulls happen to the correct branch

If a user has set the default branch to anything other than "master",
'nimble publish' fails.

This commit fixes that by ensuring that the git pull followed by git
init happens to the default branch used by upstream https://github.com/nim-lang/packages.

src/nimblepkg/publish.nim

@@ -20,6 +20,7 @@ const
   ApiKeyFile = "github_api_token"
   ApiTokenEnvironmentVariable = "NIMBLE_GITHUB_API_TOKEN"
   ReposUrl = "https://api.github.com/repos/"
+  defaultBranch = "master" # Default branch on https://github.com/nim-lang/packages
 
 proc userAborted() =
   raise newException(NimbleError, "User aborted the process.")
@@ -101,7 +102,7 @@ proc createPullRequest(a: Auth, packageName, branch: string): string =
   display("Info", "Creating PR", priority = HighPriority)
   var body = a.http.postContent(ReposUrl & "nim-lang/packages/pulls",
       body="""{"title": "Add package $1", "head": "$2:$3",
-               "base": "master"}""" % [packageName, a.user, branch])
+               "base": "$4"}""" % [packageName, a.user, branch, defaultBranch])
   var pr = parseJson(body)
   return pr{"html_url"}.getStr()
 
@@ -176,11 +177,12 @@ proc publish*(p: PackageInfo, o: Options) =
     # https://github.com/blog/1270-easier-builds-and-deployments-using-git-over-https-and-oauth
     display("Copying", "packages fork into: " & pkgsDir, priority = HighPriority)
     doCmd("git init")
+    doCmd("git checkout -b " & defaultBranch)
     doCmd("git pull https://github.com/" & auth.user & "/packages")
     # Make sure to update the fork
     display("Updating", "the fork", priority = HighPriority)
-    doCmd("git pull https://github.com/nim-lang/packages.git master")
-    doCmd("git push https://" & auth.token & "@github.com/" & auth.user & "/packages master")
+    doCmd("git pull https://github.com/nim-lang/packages.git " & defaultBranch)
+    doCmd("git push https://" & auth.token & "@github.com/" & auth.user & "/packages " & defaultBranch)
 
   if not dirExists(pkgsDir):
     raise newException(NimbleError,