Enable diffing against any commit.

Closes #309.

README.mkd

@@ -9,6 +9,7 @@ Features:
 * Ensures signs are always as up to date as possible (but without running more than necessary).
 * Quick jumping between blocks of changed lines ("hunks").
 * Stage/revert/preview individual hunks.
+* Diffs against index (default) or any commit.
 * Handles line endings correctly, even with repos that do CRLF conversion.
 * Optional line highlighting.
 * Fully customisable (signs, sign column, line highlights, mappings, extra git-diff arguments, etc).
@@ -178,6 +179,7 @@ You can customise:
 * Whether or not the sign column is shown when there aren't any signs (defaults to no)
 * The signs' colours and symbols
 * Line highlights
+* The base of the diff
 * Extra arguments for `git diff`
 * Key mappings
 * Whether or not vim-gitgutter is on initially (defaults to on)
@@ -244,6 +246,16 @@ GitGutterDeleteLine       " default: links to DiffDelete
 GitGutterChangeDeleteLine " default: links to GitGutterChangeLineDefault, i.e. DiffChange
 ```
 
+
+#### The base of the diff
+
+By default buffers are diffed against the index.  However you can diff against any commit by setting:
+
+```viml
+let g:gitgutter_diff_base = '<commit SHA>'
+```
+
+
 #### Extra arguments for `git diff`
 
 If you want to pass extra arguments to `git diff`, for example to ignore whitespace, do so like this:

autoload/gitgutter/diff.vim

@@ -63,7 +63,7 @@ function! gitgutter#diff#run_diff(realtime, preserve_full_diff)
   endif
 
   if a:realtime
-    let blob_name = ':'.gitgutter#utility#shellescape(gitgutter#utility#file_relative_to_repo_root())
+    let blob_name = g:gitgutter_diff_base.':'.gitgutter#utility#shellescape(gitgutter#utility#file_relative_to_repo_root())
     let blob_file = s:temp_index
     let buff_file = s:temp_buffer
     let extension = gitgutter#utility#extension()
@@ -88,11 +88,12 @@ function! gitgutter#diff#run_diff(realtime, preserve_full_diff)
     call setpos("']", op_mark_end)
   endif
 
-  let cmd .= 'git -c "diff.autorefreshindex=0" diff --no-ext-diff --no-color -U0 '.g:gitgutter_diff_args.' -- '
+  let cmd .= 'git -c "diff.autorefreshindex=0" diff --no-ext-diff --no-color -U0 '.g:gitgutter_diff_args.' '
+
   if a:realtime
-    let cmd .= blob_file.' '.buff_file
+    let cmd .= ' -- '.blob_file.' '.buff_file
   else
-    let cmd .= gitgutter#utility#shellescape(gitgutter#utility#filename())
+    let cmd .= g:gitgutter_diff_base.' -- '.gitgutter#utility#shellescape(gitgutter#utility#filename())
   endif
 
   if !a:preserve_full_diff && s:grep_available

doc/gitgutter.txt

@@ -117,6 +117,7 @@ You can customise:
 - The sign column's colours
 - The signs' colours and symbols
 - Line highlights
+- The base of the diff
 - Extra arguments for git-diff
 - Key mappings
 - The grep executable used
@@ -185,6 +186,14 @@ colorscheme or |vimrc|:
   GitGutterChangeDeleteLine " default: links to GitGutterChangeLineDefault
 <
 
+THE BASE OF THE DIFF
+
+By default buffers are diffed against the index.  To diff against a commit
+instead:
+>
+  let g:gitgutter_diff_base = '<commit SHA>'
+<
+
 EXTRA ARGUMENTS FOR GIT-DIFF
 
 To pass extra arguments to git-diff, add this to your |vimrc|:

plugin/gitgutter.vim

@@ -46,6 +46,7 @@ endtry
 
 call s:set('g:gitgutter_sign_modified_removed',    '~_')
 call s:set('g:gitgutter_diff_args',                  '')
+call s:set('g:gitgutter_diff_base',                  '')
 call s:set('g:gitgutter_map_keys',                    1)
 call s:set('g:gitgutter_avoid_cmd_prompt_on_windows', 1)