ok stsp

This adds the functions got_repo_cleanup_prepare() and _complete() to lock/unlock the repository to prevent multiple `gotadmin cleanup' or `git gc' operations to run in parallel. improvements and ok stsp@

improvements and ok stsp@

and use it to avoid hardcoding the digest type in a few places. ok stsp@

it abstracts over the hash type and ensures that object ids are zero'ed before their sha1 digest is written. Needed by the incoming sha256 support. ok stsp@

It will soon grow functions to deal with sha256 too. stsp@ agrees.

In preparation for wide sha256 support; stsp@ agrees. Change done mechanically with find . -iname \*.[cy] -exec sam {} + X ,x/<sha1\.h>/i/\n#include <sha2.h>

Intended mostly for the regress suite, so we'll be able to test also the ref-delta code paths. ok stsp@

gotd used 0600 (due to mkstemps(3)), gotadmin 0644; change it to 0444 since packfiles shouldn't change once created. Mirrors what git does. ok stsp@

Allows 'got commit' to use a ".diff" suffix for temporary diff files. ok op@

Needed by future gotd(8), where progress reports will be sent to a network socket, rather than a local terminal.