Merge branch 'cc/fast-import-export-signature-names' into seen

Clean up the way how signature on commit objects are exported to
and imported from fast-import stream.

* cc/fast-import-export-signature-names:
  fast-(import|export): improve on commit signature output format

diff --cc builtin/fast-export.c
Simple merge

diff --cc builtin/fast-import.c
Simple merge