mergetool: warn if ui.merge points to nonexistent tool

This adds a warning when ui.merge is configured but points to an
executable that doesn't exist. It gets printed once per fail, but that
seems to be how our other warnings about merge tools are reported.

Differential Revision: https://phab.mercurial-scm.org/D3975

mercurial/filemerge.py

@@ -208,6 +208,7 b' def _picktool(repo, ui, path, binary, sy'
     if uimerge:
         # external tools defined in uimerge won't be able to handle
         # change/delete conflicts
+        if check(uimerge, path, symlink, binary, changedelete):
         if uimerge not in names and not changedelete:
             return (uimerge, uimerge)
         tools.insert(0, (None, uimerge)) # highest priority

tests/test-merge-tools.t

@@ -1748,7 +1748,9 b' missingbinary is a merge-tool that doesn'
   missingbinary.executable=doesnotexist
   # hg update -C 1
   $ hg merge -y -r 2 --config ui.merge=missingbinary
+  couldn't find merge tool missingbinary (for pattern f)
   merging f
+  couldn't find merge tool missingbinary (for pattern f)
   revision 1
   space
   revision 0