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

@@ -183,21 +183,28 b' class CompareController(BaseRepoControll'
 
         pre_load = ["author", "branch", "date", "message"]
         c.ancestor = None
-        try:
-            c.commit_ranges = source_scm.compare(
-                source_commit.raw_id, target_commit.raw_id,
-                target_scm, merge, pre_load=pre_load)
-            if merge:
-                c.ancestor = source_scm.get_common_ancestor(
-                    source_commit.raw_id, target_commit.raw_id, target_scm)
-        except RepositoryRequirementError:
-            msg = _('Could not compare repos with different '
-                    'large file settings')
-            log.error(msg)
-            if partial:
-                return msg
-            h.flash(msg, category='error')
-            return redirect(url('compare_home', repo_name=c.repo_name))
+
+        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,
+                    target_scm, merge, pre_load=pre_load)
+                if merge:
+                    c.ancestor = source_scm.get_common_ancestor(
+                        source_commit.raw_id, target_commit.raw_id, target_scm)
+            except RepositoryRequirementError:
+                msg = _('Could not compare repos with different '
+                        'large file settings')
+                log.error(msg)
+                if partial:
+                    return msg
+                h.flash(msg, category='error')
+                return redirect(url('compare_home', repo_name=c.repo_name))
 
         c.statuses = c.rhodecode_db_repo.statuses(
             [x.raw_id for x in c.commit_ranges])