Make work tree diffs of local changes on disk display the file mode of new added files like other versions of `got diff` currently do. ok stsp@

Reported by stsp on IRC: got patch failed to set the x-bit for a new file despite got diff recording mode 755. Parse got and git diffs for this data and set file modes accordingly. Tweaked with hint from op. ok stsp@

ok stsp@

Introduce a patch_start routine that finds the next "diff" header (if there is one); the idea is to persist some state (commit id and wether it's a "git diff") while processing the content of the diff. It's needed because in the case of 'got diff' some information like the commit id are only present once at the beginning. As a consequence, the patch parser becomes slightly more robust (concatenating diffs produced by different means shouldn't confuse it anymore) and drops the support for "old" got diffs, the ones previous the introduction of the "commit -/+" header. ok tracey@

tweak and ok stsp@

ok stsp@

The idea is that got patch can simply look for a line such as: commit - abcde1234567... to find the merge base commit ID to show in diff3 conflict markers. got log -p now displays commit IDs in its diff header, instead ofl tree or blob IDs. ok op@

suggested by and ok stsp@

Parse the "blob -" metadata in diffs produced by 'got diff' and use the original file for patching. Then, use the diff3 with the current version of the file to merge the differences. This solves many failures automagically or at least turns them into a conflict. ok/improvements stsp@

ints have the advantage that their size is more likely to be the same across the various architecture supported by OpenBSD, thus introducing less possible differences. INT_MAX is still (at least) a few order of magnitudes higher than the patches we dealt with (even abnormal ones.) suggested by stsp@

ok op