[PATCH] localrepo: abort commit if a subrepo is modified and ui.commitsubs=no

[timeless]

i know people like abort early, but i'd like to suggest that abort
with all details is better.

if i have 3 subrepos and 2 are modified, i'd rather be told:

x subrepo has modifications
y subrepo has modifications
aborting due to dirty subrepos

otherwise, i try to commit (possibly writing a long message in
notepad), get an error, fix the subrepo, try to recommit, and get
another error.