Measure indent prefix on each line being commented

Previously a single indent string was used to replace all lines
which could corrupt lines using hard tabs since the indent string
used for replacement was two spaces causing the first character
of the line to be deleted.

Closes atom/language-c#1

spec/language-mode-spec.coffee

@@ -32,6 +32,10 @@ describe "LanguageMode", ->
         expect(buffer.lineForRow(6)).toBe "    //   current < pivot ? left.push(current) : right.push(current);"
         expect(buffer.lineForRow(7)).toBe "    // }"
 
+        buffer.setText('\tvar i;')
+        languageMode.toggleLineCommentsForBufferRows(0, 0)
+        expect(buffer.lineForRow(0)).toBe "\t// var i;"
+
     describe "fold suggestion", ->
       describe ".doesBufferRowStartFold(bufferRow)", ->
         it "returns true only when the buffer row starts a foldable region", ->

src/language-mode.coffee

@@ -84,8 +84,14 @@ class LanguageMode
       else
         indent = @minIndentLevelForRowRange(start, end)
         indentString = @editSession.buildIndentString(indent)
+        tabLength = @editSession.getTabLength()
+        indentRegex = new RegExp("(\t|[ ]{#{tabLength}}){#{Math.floor(indent)}}")
         for row in [start..end]
-          buffer.change([[row, 0], [row, indentString.length]], indentString + commentStartString)
+          line = buffer.lineForRow(row)
+          if indentLength = line.match(indentRegex)?[0].length
+            buffer.insert([row, indentLength], commentStartString)
+          else
+            buffer.change([[row, 0], [row, indentString.length]], indentString + commentStartString)
 
   # Folds all the foldable lines in the buffer.
   foldAll: ->