compare: using f_path shouldn't generate full range of commits.

- in case of comparing just 1 file the range of commits is not relevant.
This in addition speeds rendering the compare page by a LOT.

rhodecode/controllers/compare.py

@@ -183,6 +183,13 @@ class CompareController(BaseRepoController):
 
         pre_load = ["author", "branch", "date", "message"]
         c.ancestor = None
+
+        if c.file_path:
+            if source_commit == target_commit:
+                c.commit_ranges = []
+            else:
+                c.commit_ranges = [target_commit]
+        else:
             try:
                 c.commit_ranges = source_scm.compare(
                     source_commit.raw_id, target_commit.raw_id,