add configuration for `git mergetool` and update lightline config

.gitconfig

@@ -10,6 +10,11 @@
   eol = lf
   autocrlf = input
 
+[merge]
+  tool = vimdiff
+  conflictstyle = diff3
+  prompt = false
+
 [color]
   branch = auto
   diff = auto

.vimrc

@@ -15,6 +15,7 @@ set softtabstop=2
 set expandtab
 set listchars=tab:>-,trail:-
 set list
+set noshowmode
 syntax on
 
 filetype on
@@ -27,12 +28,17 @@ nmap n nzz
 nmap N Nzz
 
 let g:lightline = {
+      \ 'active': {
+      \   'left': [ [ 'mode', 'paste' ],
+      \             [ 'gitbranch', 'readonly', 'filename', 'modified' ] ]
+      \ },
+      \ 'component_function': {
+      \   'gitbranch': 'gitbranch#name'
+      \ },
       \ 'component': {
       \   'readonly': '%{&readonly?"":""}',
-      \ },
-      \ 'separator': { 'left': '', 'right': '' },
-      \ 'subseparator': { 'left': '', 'right': '' }
       \ }
+      \}
 
 map <C-p> :NERDTreeToggle<CR>
 " automatically close vim if the only window open is nerdtree
@@ -54,3 +60,8 @@ autocmd BufWinEnter * match ExtraWhitespace /\s\+$/
 autocmd InsertEnter * match ExtraWhitespace /\s\+\%#\@<!$/
 autocmd InsertLeave * match ExtraWhitespace /\s\+$/
 autocmd BufWinLeave * call clearmatches()
+
+" shortcuts for 3-way merge
+map <Leader>1 :diffget LOCAL<CR>
+map <Leader>2 :diffget BASE<CR>
+map <Leader>3 :diffget REMOTE<CR>