Test *_ISLINK merge environment vars

merge: provide *_ISLINK environment vars to merge helper Sets HG_MY_ISLINK, HG_OTHER_ISLINK, HG_BASE_ISLINK in environment. Without these variables, it's impossible for the merge application to know whether the 'other' and 'base' files were symlinks in their original contexts. For the purposes of the merge they are always emitted as small text files.

context: add fileflags() to avoid rebuilding manifests

run-tests.py: allow a different temporary directory to be specified

Update test for new output produced since 17ed9b9a0d03

Fix accidentally broken test.

Merge with crew.