CommitGraphWriter: Unnest generation-number progress

CommitGraphWriter: Unnest generation-number progress

The ProgressMonitor task to track the calculation of generation
numbers is nested inside the task that follows the writing of all
lines in the commit-graph. ProgressMonitor doesn't support nested
tasks and this confuses the counting.

Move the start/end of the "writing commit graph" task to the
writeCommitData section, after calculating the generation
numbers. Make that task track by commits instead of by lines.

Moving the start/end of the progress task to the chunk-writing
functions is clearer and easier to extend.

Logging of GC before:
Writing out commit-graph in 3 passes:  51% ( 9807/19358)
Computing commit-graph generation numbers: 100% (9551/9551)

Logging of GC after:
Computing commit-graph generation numbers: 100% (9551/9551)
Writing out commit-graph: 100% (9551/9551)

Change-Id: I87d69c06c9a3c7e75be12b6f0d1a63b5924e298a