Set diff base when viewing file versions with Fugitive

When you look at previous versions of a file, e.g. with Fugitive's
:0Gclog, gitgutter will set the diff base to the current version's
parent so that diff markers appear as expected.

Thannks to @rcreasi for the idea and initial implementation.

README.mkd

@@ -399,6 +399,8 @@ By default buffers are diffed against the index.  However you can diff against a
 let g:gitgutter_diff_base = '<commit SHA>'
 ```
 
+If you are looking at a previous version of a file with Fugitive (e.g. via `:0Gclog`), gitgutter sets the diff base to the parent of the current revision.
+
 This setting is ignored when the diffs are relative to the working tree.
 
 

autoload/gitgutter.vim

@@ -33,6 +33,8 @@ function! gitgutter#process_buffer(bufnr, force) abort
       return
     endif
 
+    call gitgutter#utility#set_diff_base_if_fugitive(a:bufnr)
+
     if a:force || s:has_fresh_changes(a:bufnr)
 
       let diff = 'NOT SET'

autoload/gitgutter/utility.vim

@@ -190,8 +190,20 @@ function! s:restore_shell() abort
   endif
 endfunction
 
+function! gitgutter#utility#set_diff_base_if_fugitive(bufnr)
+  let p = resolve(expand('#'.a:bufnr.':p'))
+  let ml = matchlist(p, '\v^fugitive:/.*/(\x{40,})/')
+  if !empty(ml) && !empty(ml[1])
+    let g:gitgutter_diff_base = ml[1].'^'
+  endif
+endfunction
+
 function! s:abs_path(bufnr, shellesc)
   let p = resolve(expand('#'.a:bufnr.':p'))
+
+  " Remove extra parts from fugitive's filepaths
+  let p = substitute(substitute(p, '^fugitive:', '', ''), '\v\.git/\x{40,}/', '', '')
+
   return a:shellesc ? gitgutter#utility#shellescape(p) : p
 endfunction
 

doc/gitgutter.txt

@@ -366,6 +366,9 @@ a revision instead.  For example:
     let g:gitgutter_diff_base = '<some commit SHA>'
 <
 
+If you are looking at a previous version of a file with Fugitive (e.g.
+via :0Gclog), gitgutter sets the diff base to the parent of the current revision.
+
 This setting is ignore when the diff is relative to the working tree
 (|g:gitgutter_diff_relative_to|).