bookmarks: resolve divergent bookmark when moving across a branch

This patch resolves a single divergent bookmark if a divergent bookmark exists
in the target revision and it current bookmark is not an ancestor of the target
revision, else it would already be handled by the previous patch in this
series.

Test coverage is added.

mercurial/commands.py

@@ -818,6 +818,14 b' def bookmark(ui, repo, mark=None, rev=No'
                 bmctx = repo[marks[mark]]
                 divs = [repo[b].node() for b in marks
                         if b.split('@', 1)[0] == mark.split('@', 1)[0]]
+
+                # allow resolving a single divergent bookmark even if moving
+                # the bookmark across branches when a revision is specified
+                # that contains a divergent bookmark
+                if bmctx.rev() not in anc and target in divs:
+                    bookmarks.deletedivergent(repo, [target], mark)
+                    return
+
                 deletefrom = [b for b in divs
                               if repo[b].rev() in anc or b == target]
                 bookmarks.deletedivergent(repo, deletefrom, mark)

tests/test-bookmarks.t

@@ -611,3 +611,19 b' test clearing divergent bookmarks of lin'
      Z@1                       1:925d80f479bb
      four                      3:9ba5f110a0b3
      should-end-on-two         2:db815d6d32e6
+
+test clearing only a single divergent bookmark across branches
+
+  $ hg book foo -r 1
+  $ hg book foo@1 -r 0
+  $ hg book foo@2 -r 2
+  $ hg book foo@3 -r 3
+  $ hg book foo -r foo@3
+  $ hg book
+     Z                         3:9ba5f110a0b3
+     Z@1                       1:925d80f479bb
+   * foo                       3:9ba5f110a0b3
+     foo@1                     0:f7b1eb17ad24
+     foo@2                     2:db815d6d32e6
+     four                      3:9ba5f110a0b3
+     should-end-on-two         2:db815d6d32e6