tests/histedit-helpers.sh

tests: add a wrapper to run fsmonitor tests

This script does a bunch of non-trivial configuration work: in particular, it
sets up an isolated instance of Watchman which isn't affected by global state
and can be torn down on completion.

This script also sets the HGFSMONITOR_TESTS environment variable, which hghave
will use in the next patch to allow gating on whether fsmonitor is enabled.

With fsmonitor enabled, there appear to be a number of failures in the test
suite. It's not yet clear to me why they're happening, but if someone would
like to jump in and fix some of them I hope this will be helpful for that.

fixbundle() {
    grep -v 'saving bundle' | grep -v 'saved backup' | \
        grep -v added | grep -v adding | \
        grep -v "unable to find 'e' for patching" | \
        grep -v "e: No such file or directory" | \
    cat
}