largefiles: introduce "_lfcommithooks" to abstract pre-committing procedures
This changes allows to customize pre-committing procedures according
to conditions.
This patch uses "stack" with an element instead of flag like
"_isXXXXing" or so, because:
- the former works correctly even when customizations are nested, and
- ensuring at least one element can ignore empty check
#require test-repo pyflakes
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate 'set:**.py or grep("^!#.*python")' 2>/dev/null \
> | xargs pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
tests/filterpyflakes.py:58: undefined name 'undefinedname'