sourcebuild: allow for syncing existing repositories

[mara004]

If we are building from source and the repositories of DepotTools and PDFium are already present from previous runs, the current behaviour is to do nothing and re-use the existing repository as-is.
It would make sense if our build script could automate syncing/updating the repository. However, I think this should be opt-in to allow for building a custom version of PDFium, and to prevent inadvertent loss of unstaged changes.

The update strategy for the PDFium repository could either be done manually:

# clean up uncommitted changes (i. e. patches from the previous build)
run_cmd(f"git reset --hard HEAD", cwd=PDFiumDir)

# the `build/` directory contains an independent git repository, so reset that, too
run_cmd(f"git reset --hard HEAD", cwd=join(PDFiumDir,'build'))

# remove the `resources.rc` file that is not affected by `git reset`
os.remove(join(PDFiumDir,'resources.rc'))

# sync
run_cmd("gclient sync --no-history --shallow", cwd=WorkDir)

Or probably a lot easier with gclient:

run_cmd("gclient revert", cwd=WorkDir)
run_cmd("gclient sync", cwd=WorkDir)  # not sure whether revert already includes a sync