Clarify commit error message for unmerged files

Clarify commit error message for unmerged files

Currently, trying to use git-commit with unmerged files in the index
will show the message "Error building trees", which can be a bit
obscure to the end user. This patch makes the error message clearer, and
consistent with what git-write-tree reports in a similar situation.

Signed-off-by: Rafael Garcia-Suarez <redacted>
Signed-off-by: Shawn O. Pearce <redacted>