vcs: Deprecate generic missing commit merge error reason.

There are two more specific error reasons (target/source ref missing)
which should be used instead.

rhodecode/lib/vcs/backends/base.py

@@ -94,8 +94,9 b' class MergeFailureReason(object):'
     # The target repository is locked
     TARGET_IS_LOCKED = 7
 
+    # Deprecated, use MISSING_TARGET_REF or MISSING_SOURCE_REF instead.
     # A involved commit could not be found.
-    MISSING_COMMIT = 8
+    _DEPRECATED_MISSING_COMMIT = 8
 
     # The target repo reference is missing.
     MISSING_TARGET_REF = 9

rhodecode/model/pull_request.py

@@ -95,7 +95,7 b' class PullRequestModel(BaseModel):'
         MergeFailureReason.TARGET_IS_LOCKED: lazy_ugettext(
             'This pull request cannot be merged because the target repository'
             ' is locked.'),
-        MergeFailureReason.MISSING_COMMIT: lazy_ugettext(
+        MergeFailureReason._DEPRECATED_MISSING_COMMIT: lazy_ugettext(
             'This pull request cannot be merged because the target or the '
             'source reference is missing.'),
         MergeFailureReason.MISSING_TARGET_REF: lazy_ugettext(