Update button titles in merge conflicts widget header.

app/assets/javascripts/merge_conflict_data_provider.js.es6

 const HEAD_HEADER_TEXT    = 'HEAD//our changes';
 const ORIGIN_HEADER_TEXT  = 'origin//their changes';
 
+const HEAD_BUTTON_TITLE   = 'Use ours';
+const ORIGIN_BUTTON_TITLE = 'Use theirs';
+
 window.MergeConflictDataProvider = class MergeConflictDataProvider {
 
   getInitialData() {
@@ -275,6 +278,7 @@ window.MergeConflictDataProvider = class MergeConflictDataProvider {
     return {
       id          : id,
       richText    : HEAD_HEADER_TEXT,
+      buttonTitle : HEAD_BUTTON_TITLE,
       type        : 'new',
       section     : 'head',
       isHeader    : true,
@@ -289,6 +293,7 @@ window.MergeConflictDataProvider = class MergeConflictDataProvider {
     return {
       id          : id,
       richText    : ORIGIN_HEADER_TEXT,
+      buttonTitle : ORIGIN_BUTTON_TITLE,
       type        : 'old',
       section     : 'origin',
       isHeader    : true,

app/views/projects/merge_requests/conflicts.html.haml

@@ -62,7 +62,8 @@
                       'selected': line.isSelected, |
                       'unselected': line.isUnselected}"}
                       %strong {{line.richText}}
-                      %button.btn{"@click" => "handleSelected(line.id, line.section)"} Use this
+                      %button.btn{"@click" => "handleSelected(line.id, line.section)"}
+                        {{line.buttonTitle}}
 
                   %template{"v-if" => "!line.isHeader"}
                     %td.diff-line-num.old_line{":class" => "{ |
@@ -113,7 +114,8 @@
                   %td.diff-line-num.header
                   %td.line_content.header
                     %strong {{{line.richText}}}
-                    %button.btn{"@click" => "handleSelected(line.id, line.section)"} Use this
+                    %button.btn{"@click" => "handleSelected(line.id, line.section)"}
+                      {{line.buttonTitle}}
 
     .content-block.oneline-block.files-changed
       %strong.resolved-count {{resolvedCount}}