Merge branch 'jk/blame-fixes' into maint

"git blame --porcelain" misidentified the "previous" <commit, path>
pair (aka "source") when contents came from two or more files.

* jk/blame-fixes:
blame: output porcelain "previous" header for each file
blame: handle --no-abbrev
blame: fix alignment with --abbrev=40

diff --cc builtin/blame.c
Simple merge