Fix toolstate history format

[Mark-Simulacrum]

We were inserting before the existing newline, so we should prepend it
not append it to our inserted string.

[Centril]

Let's rubber-stamp & see if it fixes things.

@bors r+ rollup

[bors]

📌 Commit 1787b2b has been approved by Centril

[bors]

🌲 The tree is currently closed for pull requests below priority 100, this pull request will be tested once the tree is reopened

[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

2019-12-19T18:50:19.7531310Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-12-19T18:50:19.7543506Z ##[command]git config gc.auto 0
2019-12-19T18:50:19.7545371Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-12-19T18:50:19.7546886Z ##[command]git config --get-all http.proxy
2019-12-19T18:50:19.7614500Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/67432/merge:refs/remotes/pull/67432/merge
---
2019-12-19T19:00:32.2664211Z configure: build.locked-deps    := True
2019-12-19T19:00:32.2664272Z configure: llvm.ccache          := sccache
2019-12-19T19:00:32.2664466Z configure: build.cargo-native-static := True
2019-12-19T19:00:32.2664652Z configure: dist.missing-tools   := True
2019-12-19T19:00:32.2664902Z configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
2019-12-19T19:00:32.2664996Z configure: writing `config.toml` in current directory
2019-12-19T19:00:32.2665053Z configure: 
2019-12-19T19:00:32.2665280Z configure: run `python /checkout/x.py --help`
2019-12-19T19:00:32.2665324Z configure: 
---
2019-12-19T19:09:07.9230770Z Build completed successfully in 0:01:43
2019-12-19T19:09:07.9239930Z + /scripts/validate-toolstate.sh
2019-12-19T19:09:07.9301747Z Cloning into 'rust-toolstate'...
2019-12-19T19:09:08.4800738Z Traceback (most recent call last):
2019-12-19T19:09:08.4801464Z   File "../../src/tools/publish_toolstate.py", line 303, in <module>
2019-12-19T19:09:08.4801894Z     cur_datetime
2019-12-19T19:09:08.4801993Z   File "../../src/tools/publish_toolstate.py", line 182, in update_latest
2019-12-19T19:09:08.4802657Z     for os in ['windows', 'linux']
2019-12-19T19:09:08.4802791Z   File "../../src/tools/publish_toolstate.py", line 182, in <dictcomp>
2019-12-19T19:09:08.4803003Z     for os in ['windows', 'linux']
2019-12-19T19:09:08.4803053Z   File "../../src/tools/publish_toolstate.py", line 111, in read_current_status
2019-12-19T19:09:08.4803287Z     (commit, status) = line.split('\t', 1)
2019-12-19T19:09:08.4803340Z ValueError: need more than 1 value to unpack
2019-12-19T19:09:08.4850113Z   local time: Thu Dec 19 19:09:08 UTC 2019
2019-12-19T19:09:08.7477272Z   network time: Thu, 19 Dec 2019 19:09:08 GMT
2019-12-19T19:09:08.7481638Z == end clock drift check ==
2019-12-19T19:09:10.7489649Z 
2019-12-19T19:09:10.7489649Z 
2019-12-19T19:09:10.7645878Z ##[error]Bash exited with code '1'.
2019-12-19T19:09:10.7681477Z ##[section]Starting: Checkout
2019-12-19T19:09:10.7683449Z ==============================================================================
2019-12-19T19:09:10.7683525Z Task         : Get sources
2019-12-19T19:09:10.7683573Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

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