Fix infinite loop in _build_line_mappings when reaching beginning of history

Author: nh2

gitbrowse/git.py

@@ -182,8 +182,8 @@ def _build_line_mappings(self, start, finish):
         sections = []
 
         # Skip initial headers: They don't interest us.
-        line = ''
-        while not line.startswith('@@'):
+        line = p.readline()
+        while line != '' and not line.startswith('@@'):
             line = p.readline()
 
         while line: