Get initial/updated shas from `git pull` output.

Instead of creating a vundle_update tag to compare changes between update
and HEAD we now use the output from `git pull` to get initial/updated shas.

autoload/vundle/installer.vim

@@ -206,7 +206,6 @@ func! s:sync(bang, bundle) abort
       let cmd = substitute(cmd, '^cd ','cd /d ','')  " add /d switch to change drives
       let cmd = '"'.cmd.'"'                          " enclose in quotes
     endif
-    call s:add_update_tag(a:bundle)
   else
     let cmd = 'git clone '.a:bundle.uri.' '.shellescape(a:bundle.path())
   endif
@@ -227,7 +226,7 @@ func! s:sync(bang, bundle) abort
     return 'todate'
   endif
 
-  call add(g:updated_bundles, a:bundle)
+  call s:add_to_updated_bundles(out, a:bundle)
   return 'updated'
 endf
 
@@ -235,9 +234,12 @@ func! s:system(cmd) abort
   return system(a:cmd)
 endf
 
-func! s:add_update_tag(bundle) abort
-  call s:system('cd '.shellescape(a:bundle.path()).
-        \  ' && git tag -a vundle_update -m "Last Vundle Update" -f')
+func! s:add_to_updated_bundles(out, bundle) abort
+  let git_pull_shas = matchlist(a:out, 'Updating \(\w\+\)..\(\w\+\)')
+  let initial_sha = git_pull_shas[1]
+  let updated_sha = git_pull_shas[2]
+
+  call add(g:updated_bundles, [initial_sha, updated_sha, a:bundle])
 endfunc
 
 func! s:log(str) abort

autoload/vundle/scripts.vim

@@ -34,16 +34,19 @@ func! s:view_log()
 endf
 
 func! s:create_changelog() abort
-  for bundle in g:updated_bundles
+  for bundle_data in g:updated_bundles
+    let initial_sha = bundle_data[0]
+    let updated_sha = bundle_data[1]
+    let bundle      = bundle_data[2]
+
     let updates = system('cd '.shellescape(bundle.path()).
-          \              ' && git log --pretty=format:"%s   %an, %ar" --graph'.
-          \              ' vundle_update..HEAD')
+          \              ' && git log --pretty=format:"%s   %an, %ar" --graph '.
+          \               initial_sha.'..'.updated_sha)
     call add(g:vundle_changelog, '')
     call add(g:vundle_changelog, 'Updated Bundle: '.bundle.name)
 
     if bundle.uri =~ "https://github.com"
-      let update_sha = system('cd '.shellescape(bundle.path()).' && git rev-list -1 vundle_update')[0:9]
-      call add(g:vundle_changelog, 'Compare at: '.bundle.uri[0:-5].'/compare/'.update_sha.'...HEAD')
+      call add(g:vundle_changelog, 'Compare at: '.bundle.uri[0:-5].'/compare/'.initial_sha.'...'.updated_sha)
     endif
 
     for update in split(updates, '\n')