Handle file which does not exist in diff base

If g:gitgutter_diff_base has been set and the file being processed does
not exist in that branch/commit, ensure that every line is marked as
added - this is how git-diff behaves.

Fixes airblade#855.

autoload/gitgutter.vim

@@ -46,6 +46,8 @@ function! gitgutter#process_buffer(bufnr, force) abort
         call gitgutter#debug#log('Not tracked: '.gitgutter#utility#file(a:bufnr))
       catch /gitgutter assume unchanged/
         call gitgutter#debug#log('Assume unchanged: '.gitgutter#utility#file(a:bufnr))
+      catch /gitgutter file unknown in base/
+        let diff = gitgutter#diff#hunk_header_showing_every_line_added(a:bufnr)
       catch /gitgutter diff failed/
         call gitgutter#debug#log('Diff failed: '.gitgutter#utility#file(a:bufnr))
         call gitgutter#hunk#reset(a:bufnr)

autoload/gitgutter/diff.vim

@@ -81,6 +81,20 @@ function! gitgutter#diff#run_diff(bufnr, from, preserve_full_diff) abort
     throw 'gitgutter assume unchanged'
   endif
 
+  " If we are diffing against a specific branch/commit, handle the case
+  " where a file exists on the current branch but not in/at the diff base.
+  " We have to handle it here because the approach below (using git-show)
+  " doesn't work for this case.
+  if !empty(g:gitgutter_diff_base)
+    let index_name = gitgutter#utility#get_diff_base(a:bufnr).':'.gitgutter#utility#repo_path(a:bufnr, 1)
+    let cmd = g:gitgutter_git_executable.' '.g:gitgutter_git_args.' --no-pager show '.index_name
+    let cmd = gitgutter#utility#cd_cmd(a:bufnr, cmd)
+    call gitgutter#utility#system(cmd)
+    if v:shell_error
+      throw 'gitgutter file unknown in base'
+    endif
+  endif
+
   " Wrap compound commands in parentheses to make Windows happy.
   " bash doesn't mind the parentheses.
   let cmd = '('
@@ -376,6 +390,12 @@ function! gitgutter#diff#hunk_diff(bufnr, full_diff, ...)
 endfunction
 
 
+function! gitgutter#diff#hunk_header_showing_every_line_added(bufnr)
+  let buf_line_count = getbufinfo(a:bufnr)[0].linecount
+  return '@@ -0,0 +1,'.buf_line_count.' @@'
+endfunction
+
+
 function! s:write_buffer(bufnr, file)
   let bufcontents = getbufline(a:bufnr, 1, '$')
 

test/test_gitgutter.vim

@@ -348,6 +348,21 @@ function Test_untracked_file_square_brackets_within_repo()
 endfunction
 
 
+function Test_file_unknown_in_base()
+  let starting_branch = system('git branch --show-current')
+  let starting_branch = 'main'
+  call system('git checkout -b some-feature')
+  let tmp = 'file-on-this-branch-only.tmp'
+  call system('echo "hi" > '.tmp.' && git add '.tmp)
+  execute 'edit '.tmp
+  let g:gitgutter_diff_base = starting_branch
+  GitGutter
+  let expected = [{'lnum': 1, 'name': 'GitGutterLineAdded', 'group': 'gitgutter', 'priority': 10}]
+  call s:assert_signs(expected, tmp)
+  let g:gitgutter_diff_base = ''
+endfunction
+
+
 function Test_hunk_outside_noop()
   5
   GitGutterStageHunk