Improve char width computation (#593)

Char width computation didn't work as expected when the first character
of the NSTextView was an emoji. The `maximumAdvancement` returned the
length of the emoji, placing the margins at unexpected position.

This commit improves the computation by using a fixed character (`x` has
been chosen, arbitrarily). The margin will thus be placed at 50
non-emoji characters.

Author: pbernery

GitUpKit/Utilities/GIAppKit.m

@@ -174,7 +174,10 @@ - (void)drawRect:(NSRect)dirtyRect {
   if ([[NSUserDefaults standardUserDefaults] boolForKey:GICommitMessageViewUserDefaultKey_ShowMargins]) {
     NSRect bounds = self.bounds;
     CGFloat offset = self.textContainerOrigin.x + self.textContainerInset.width + self.textContainer.lineFragmentPadding;
-    CGFloat charWidth = self.font.maximumAdvancement.width;  // TODO: Is this the most reliable way to get the character width of a fixed-width font?
+    CGFloat charWidth = [@"x" sizeWithAttributes:@{
+                           NSFontAttributeName : self.font
+                         }]
+                             .width;
     CGContextRef context = [[NSGraphicsContext currentContext] graphicsPort];
 
     CGContextSaveGState(context);