The HgToGitConverter waits for child processes indefinitely

When the HgToGitConverter runs various git and hg commands, it uses Process.waitFor without specifying any timeout. If for some reason the child process would hang, this means that we will also effectively hang. This has happened occasionally, for example, this is the last entry of a log:

[2019-08-06T17:10:42Z][74][finer] Starting 'git fast-import'

Timeouts should be added at the appropriate places (can probably just set them to something large, like 12 hours).