fixes send.sh test fallout on arm64 and should also fix i386 ok op@

ok op

This adds a set of functions to abstract over SHA1Init, SHA1Update, SHA1Final, their respective SHA256 variants and how to compare digests. Replace all the SHA1*() usage with the new APIs. It's a preparatory step for sha256 handling. 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>

Patch by Josiah Frentsos, thanks!

Don't assume commit info is always written. This is only true when diffing a commit against (one of) its direct parent(s). Otherwise we perform an invalid read on a tmp got_diff_line array and end up passing bogus offsets to fseeko(). Bug found and fixed by stsp with a minor tweak by me. ok jamsek for stsp's initial diff ok stsp@

This fixes the (harmless) errors raised by gitconfig.c on indented comment lines and adds a test case for it. Reported by James Cook, thanks! ok stsp@

The first example is meant to show the default values for all the options. Sync with reality.

cmd_merge is the only subcommand that may create commits that doesn't parse the git config files, thusly failing for users without GOT_AUTHOR defined. Problem reported by James Cook who also provided an initial test case, thanks! ok jamsek