unshelve: disable unshelve during merge (issue5123)

As stated in the issue5123, unshelve can destroy the second parent of
the context when tried to unshelve with an uncommitted merge. This
patch makes unshelve to abort when called with an uncommitted merge.

See how shelve.mergefiles works. Commit structure looks like this:
```
... -> pctx -> tmpwctx -> shelvectx
                  /
                 /
        second
        merge parent

pctx = parent before merging working context(first merge parent)
tmpwctx = commited working directory after merge(with two parents)
shelvectx = shelved context
```
shelve.mergefiles first updates to pctx then it reverts shelvectx to pctx with:

```
cmdutil.revert(ui, repo, shelvectx, repo.dirstate.parents(),
                       *pathtofiles(repo, files),
                       **{'no_backup': True})
```

Reverting tmpwctx files that were merged from second parent to pctx makes them
added because they are not in pctx.

Changing this revert operation is crucial to restore parents after unshelve.
This is a complicated issue as this is not fixing a regression. Thus, for the
time being, unshelve during an uncommitted merge can be aborted.

(Details taken from http://mercurial.808500.n3.nabble.com/PATCH-V3-shelve-restore-parents-after-unshelve-issue5123-tt4036858.html#a4037408)

Differential Revision: https://phab.mercurial-scm.org/D6169

hgext/shelve.py

@@ -978,6 +978,12 b' def _dounshelve(ui, repo, *shelved, **op'
             return unshelvecontinue(ui, repo, state, opts)
     elif len(shelved) > 1:
         raise error.Abort(_('can only unshelve one change at a time'))
+
+    # abort unshelve while merging (issue5123)
+    parents = repo[None].parents()
+    if len(parents) > 1:
+        raise error.Abort(_('cannot unshelve while merging'))
+
     elif not shelved:
         shelved = listshelves(repo)
         if not shelved:

tests/test-shelve.t

@@ -1111,3 +1111,49 b' Keep active bookmark while (un)shelving '
      test                      (4|13):33f7f61e6c5e (re)
 
   $ cd ..
+
+Abort unshelve while merging (issue5123)
+----------------------------------------
+
+  $ hg init issue5123
+  $ cd issue5123
+  $ echo > a
+  $ hg ci -Am a
+  adding a
+  $ hg co null
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo > b
+  $ hg ci -Am b
+  adding b
+  created new head
+  $ echo > c
+  $ hg add c
+  $ hg shelve
+  shelved as default
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg co 1
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge 0
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+-- successful merge with two parents
+  $ hg log -G
+  @  changeset:   1:406bf70c274f
+     tag:         tip
+     parent:      -1:000000000000
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     b
+  
+  @  changeset:   0:ada8c9eb8252
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     a
+  
+-- trying to pull in the shelve bits
+-- unshelve should abort otherwise, it'll eat my second parent.
+  $ hg unshelve
+  abort: cannot unshelve while merging
+  [255]
+
+  $ cd ..