subrepo: drop arguments unsupported by old git

subrepo: work around old git's remote naming scheme Older git versions use 'origin/master' Current git uses 'remotes/origin/master'

subrepo: use environment variable instead of git commit's --date Older git versions do not have a --date flag.

subrepo: parse git status's human-readable output Older git versions do not have a machine-readable output, but the default format has not changed over time.

subrepo: use subprocess's cwd instead of git's --work-tree Some older git commands (e.g. git merge) do not properly recognize the --work-tree argument, so abstract that away from git.

test-subrepo-git.t: make gitroot pushable earlier Older git versions get confused when the git repo becomes bare.

test-subrepo-git.t: silence git output Older git versions have different ideas of what output is considered quiet, so the tests can be more version-blind.

bdiff: Fix bogus NULL return

bdiff: dynamically allocate hunks Make the hunk list a singly-linked list rather than a large array.

merge with stable