Use Semi Bold font in commit details subject line

This allows clearly differentiating between the subject line and body.

Using Semi Bold (600) weight also makes it easier to see the subject
among the rest of the UI, without overpowering it, unlike Bold (700).

Co-authored-by: Hari Rana <theevilskeleton@riseup.net>

gitg/resources/ui/style.css

@@ -126,6 +126,12 @@ GitgHistoryRefRow {
 	font-weight: bold;
 }
 
+.subject-label-semi-bold {
+  /* In the main commit diff details header, instead of using full bold (700),
+     use semi-bold for the 1st line to avoid overpowering the rest of the UI. */
+  font-weight: 600;
+}
+
 .sha1-label {
 	font-size: 0.8em;
 	color: @insensitive_fg_color;

libgitg/resources/ui/gitg-diff-view-commit-details.ui

@@ -130,6 +130,9 @@
             <property name="wrap">True</property>
             <property name="selectable">True</property>
             <property name="use_markup">True</property>
+            <style>
+              <class name="subject-label-semi-bold" />
+            </style>
           </object>
           <packing>
             <property name="left_attach">0</property>