git: Replace a few porcelain commands with plumbing commands

[jerry-skydio]

• diff --cached -> diff-index --cached (index vs tree, immune to the
  stale-mtime quirk)
• diff ~ -> diff-tree
• stat/shortstat conversions add -M since porcelain detects renames by
  default; --no-renames cases don't.
• -r is added so subdirectories are recursed.

The worktree comparison in get_has_unstaged is intentionally left as
porcelain: diff-index returns 1 on a refreshed mtime with no content
change, which would report spurious unstaged changes.

• branch --show-current -> symbolic-ref -q --short HEAD. The -q with
  raiseonerror=False preserves the porcelain behavior of returning empty
  on a detached HEAD instead of erroring.
• show : -> cat-file -p, the plumbing equivalent for reading
  a blob, used when dumping conflict markers.

[jerry-skydio]

Reviews in this chain:
└#263 git: Replace a few porcelain commands with plumbing commands

[jerry-skydio]

#	head	base	diff	date	summary
0	44ac06d0	e03937b2	diff	Jun 19 14:10 PM	2 files changed, 33 insertions(+), 8 deletions(-)
1	f58f9458	e03937b2	diff	Jun 19 14:11 PM	1 file changed, 2 deletions(-)
2	81f1de0f	e03937b2	diff	Jun 19 14:19 PM	1 file changed, 2 insertions(+), 16 deletions(-)