errors: raise StateError on uncommitted changes when merge starts

Differential Revision: https://phab.mercurial-scm.org/D9393

mercurial/merge.py

@@ -1912,7 +1912,7 b' def _update('
                         hint=_(b"use 'hg update' or check 'hg heads'"),
                     )
             if not force and (wc.files() or wc.deleted()):
-                raise error.Abort(
+                raise error.StateError(
                     _(b"uncommitted changes"),
                     hint=_(b"use 'hg status' to list changes"),
                 )

tests/test-merge1.t

@@ -282,7 +282,7 b' merge fails'
   $ hg merge 2
   abort: uncommitted changes
   (use 'hg status' to list changes)
-  [255]
+  [20]
 merge expected!
   $ hg merge -f 2
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
@@ -319,7 +319,7 b' merge of b should fail'
   $ hg merge 2
   abort: uncommitted changes
   (use 'hg status' to list changes)
-  [255]
+  [20]
 merge of b expected
   $ hg merge -f 2
   merging b