Add doc for blame and history UI functionality

While the functionality was created ~ 7 years ago, it was never doc'd

doc/user/project/repository/git_blame.md

+---
+type: reference, howto
+description: "Documentation on Git file blame."
+---
+
+# Git file blame
+
+> [Introduced](https://git.sphere.ly/staff/publicgitlab/commit/39c657930625ddc3ac8a921f01ffc83acadce68f) in GitLab 2.5
+
+[Git blame](https://git-scm.com/docs/git-blame) provides more information
+about every line in a file, including the last modified time, author, and
+commit hash.
+
+You can find the **Blame** button with each file in a project.
+
+![File blame button](img/file_blame_button_v12_6.png "Blame button")
+
+When you select the **Blame** button, you'll see a screen with the
+noted information:
+
+![Git blame output](img/file_blame_output_v12_6.png "Blame button output")
+
+If you hover over a commit in the UI, you'll see a precise date and time
+for that commit.
+
+## Associated `git` command
+
+If you're running `git` from the command line, the equivalent command is
+`git blame <filename>`. For example, if you want to find `blame` information
+about a `README.md` file in the local directory, run the following command:
+
+```bash
+git blame README.md
+```
+
+You'll see output similar to the following, which includes the commit time
+in UTC format:
+
+```bash
+62e2353a (Achilleas Pipinellis     2019-07-11 14:52:18 +0300   1) [![build status](https://gitlab.com/gitlab-org/gitlab-docs/badges/master/build.svg)](https://gitlab.com/gitlab-com/gitlab-docs/commits/master)
+fb0fc7d6 (Achilleas Pipinellis     2016-11-07 22:21:22 +0100   2)
+^764ca75 (Connor Shea              2016-10-05 23:40:24 -0600   3) # GitLab Documentation
+^764ca75 (Connor Shea              2016-10-05 23:40:24 -0600   4)
+0e62ed6d (Mike Jang                2019-11-26 21:44:53 +0000   5) This project hosts the repository used to generate the GitLab
+0e62ed6d (Mike Jang                2019-11-26 21:44:53 +0000   6) documentation website and deployed to https://docs.gitlab.com. It uses the
+```
+
+## File blame through the API
+
+You can also get this information over the [Git file blame REST API](../../../api/repository_files.md#get-file-blame-from-repository).

doc/user/project/repository/git_history.md

+---
+type: reference, howto
+description: "Documentation on Git file history."
+---
+
+# Git file history
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/blob/9ba1224867665844b117fa037e1465bb706b3685/app/controllers/commits_controller.rb) in GitLab 0.8.0
+
+Git file History provides information about the commit history associated
+with a file.
+
+You can find the **History** button with each file in a project.
+
+![File history button](img/file_history_button_v12_6.png "History button")
+
+When you select the **History** button, you'll see a screen with the
+noted information:
+
+![Git log output](img/file_history_output_v12_6.png "History button output")
+
+If you hover over a commit in the UI, you'll see a precise date and time
+that commit was last modified.
+
+## Associated `git` command
+
+If you're running `git` from the command line, the equivalent command
+is `git log <filename>`. For example, if you want to find `history`
+information about a `README.md` file in the local directory, run the
+following command:
+
+```bash
+git log README.md
+```
+
+You'll see output similar to the following, which includes the commit
+time in UTC format:
+
+```bash
+commit 0e62ed6d9f39fa9bedf7efc6edd628b137fa781a
+Author: Mike Jang <mjang@gitlab.com>
+Date:   Tue Nov 26 21:44:53 2019 +0000
+
+    Deemphasize GDK as a doc build tool
+
+commit 418879420b1e3a4662067bd07b64bb6988654697
+Author: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
+Date:   Mon Nov 4 19:58:27 2019 +0100
+
+    Fix typo
+
+commit 21cc1fef11349417ed515557748369cfb235fc81
+Author: Jacques Erasmus <jerasmus@gitlab.com>
+Date:   Mon Oct 14 22:13:40 2019 +0000
+
+    Add support for modern JS
+
+    Added rollup to the project
+
+commit 2f5e895aebfa5678e51db303b97de56c51e3cebe
+Author: Achilleas Pipinellis <axil@gitlab.com>
+Date:   Fri Sep 13 14:03:01 2019 +0000
+
+    Remove gitlab-foss Git URLs as we don't need them anymore
+
+    [ci skip]
+```

doc/user/project/repository/index.md

@@ -48,6 +48,8 @@ it's easier to do so [via GitLab UI](web_editor.md):
 - [File templates](web_editor.md#template-dropdowns)
 - [Create a directory](web_editor.md#create-a-directory)
 - [Start a merge request](web_editor.md#tips)
+- [Find file history](git_history.md)
+- [Identify changes by line (Git blame)](git_blame.md)
 
 **From the command line:**