Failed recursive merging can silently lose date in StGIT

Failed recursive merging can silently lose date in StGIT

The logic in git.merge(recursive = True) is that if
git-merge-recursive fails, deal with the unmerged entries via diff3
and interactive merge (if 'autoimerge' is set to 'yes' in
config). There is a situation, however, when git-merge-recursive fails
because of an untracked file that would be overwritten by the
merge. In this case, git-merge-recursive aborts the operation but
without any unmerged entries in the index. StGIT considers that the
merge lead to an empty patch without reporting any error.

This commit makes it a bit safer by detecting whether there are
unmerged index entries and, if not, further raises an error informing
the user of the problem. The patch is still pushed as empty but the
user is informed of the failure and the possibility of running 'push
--undo'.

A better solution, but which require a bit more work, is to
distinguish between the merge failures and, in this specific case,
abort the push completely and revert the patch to its original state
(and popped from the stack).

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>