diff: use fctx.isbinary() to test binary
The end goal is to avoid calling fctx.data() when unnecessary. For example,
if diff.nobinary=1 and files are binary, the expected behavior is to print
"Binary file has changed". That could avoid reading fctx.data() sometimes.
This is mainly to enable an external LFS extension to skip expensive binary
file loading sometimes (read: most of the time with diff.nobinary=1 and
diff.text=0), without any behavior changes to mercurial (i.e. whether a file
is LFS or not does not change any behavior, LFS could be 100% transparent to
users).
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).