Works similar to how several Git forges authenticate their webhooks. Feature requested by dch@freebsd. Help from tb@ regarding which libcrypto calls to use, thanks! ok op@ (with some follow-up tweaks suggested)

tweak + ok op@, "seems good to me" (webpage review) stsp@

This matches the output of "got diff -d". ok jamsek

Suggested by naddy: display diffstat in diff views of arbitrary commits. For the sake of consistency, show it in tree and blob diffs now too. Adjust and grow regress to cover the change. ok stsp@

This helps identify diffs when several have been written. Suggested by stsp on IRC. ok stsp@

Write the current diff view to a temporary patch file in /tmp and report the pathname to the status line. discussed with naddy input and ok stsp@

This pointer is owned by the caller and is freed in error paths via close_diff_view() ok stsp@

Since we may update the fileindex, the worktree must be preemptively locked exclusively. It's an old thing, been there since the start. ok stsp@

ok op@

This exends got_worktree_path_info() to resemble the "transaction"-like interface we have for rebase, patch etc. There's a prepare() routine that returns the fileindex, and locks the worktree, and a complete() function to free the fileinedx and release the lock. This way, we can open only once the fileindex in cmd_info() and have the chance to ask for its version. ok stsp@

No functional change intended.

In my early sha256 work I've used `idlen' to parametrize the digest length, but that's ambiguous since it could refer either to the digest length (in binary form) or the digest _string_ length (hexadecimal). So, change the few offenders to either digest_len or digest_string_len, which is a "naming scheme" I've already used in the rest of the tree. ok stsp@

This changes the search_packidx, match_packed_object and get_packfile_info routines to skip over lonely packidx. These seems to be generated occasionally by 'git fetch' over HTTP/S. Instead of dealing with this situation in gotwebd, which is fragile, attempt to do it at the lib/ level. `gotadmin cleanup' will still complain about these lonely packidx and `gotadmin cleanup -p' is still required. discussed with and ok stsp@

Later we try to see to -digest_len from the end of the file, and this on 32bit arches has some issues: size_t is 32bit and being unsigned becomes a huge number that then is casted to a off_t (signed, and larger) and makes us seek very, very far from the end of file. Then, read(2) returns zero because we're at end of file and trying to send packs to gotd fails with "I/O error". This was introduced when SHA1_DIGEST_LENGTH was converted to digest_len. ok jamsek, stsp

Results in the very same error but takes less characters to build. No functional change intended.