Use upstream ref if possible

plugin/vim-gh-line.vim

@@ -231,11 +231,31 @@ func! s:Action(remote_url, action)
   endif
 endfunc
 
+func! s:BranchName(cdDir, ref)
+  return trim(system(a:cdDir . 'git rev-parse --abbrev-ref ' . a:ref))
+endfunc
+
+func! s:CommitRef(cdDir)
+  let l:upstreamBranch = system(a:cdDir . 'git rev-parse --abbrev-ref --symbolic-full-name @{u}')
+  if v:shell_error == 0
+    return l:upstreamBranch
+  else
+    let l:currentBranch = s:BranchName(a:cdDir, 'HEAD')
+    let l:branch = input('Please provide a remote branch (defaults to ' . l:currentBranch . '): ')
+    if len(l:branch) == 0
+      return l:currentBranch
+    endif
+
+    return l:branch
+  endif
+endfunc
+
 func! s:Commit(cdDir)
+  let l:ref = s:CommitRef(a:cdDir)
   if exists('g:gh_use_canonical') && g:gh_use_canonical > 0
-    return system(a:cdDir . 'git rev-parse HEAD')
+    return system(a:cdDir . 'git rev-parse ' . l:ref)
   else
-    return system(a:cdDir . 'git rev-parse --abbrev-ref HEAD')
+    return s:BranchName(l:ref)
   endif
 endfunc
 

test/vim-gh-line_test.vim

@@ -93,7 +93,7 @@ func! s:testCommit(sid)
 
     let g:gh_use_canonical = 0
     let l:act = s:callWithSID(a:sid, 'Commit', l:cdDir)
-    call assert_match(l:branch, l:act,
+    call assert_equal(l:branch, l:act,
         \ 'Expected to find branch name in the Commit output ')
     unlet g:gh_use_canonical
 endfunction