Support post-merge and post-commit git hooks

[stealthybox]

• Gogs version: 0.11.34.1122
• Git version: 1.8.3.1
• Operating system: CentOS Linux release 7.4.1708 (Core)
• Database:
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gogs.io:
  • Yes (provide example URL)
  • No
  • Not relevant

Description

It would be useful to support post-merge and other git hooks intended for local use on the gogs remote.

My use-case would be to run a git hook when a pull-request is merged.

This is part of the use case in #96 -- it's possible to run hooks when a user pushes to the repo's remote but not when gogs modifies the repo locally such when merging a pull request or modifying files in the web UI.

post-commit may be useful for resolving #4338.

However, I don't believe these hooks are supported on bare repositories.
I tried manually adding post-commit and post-merge hooks to the gogs bare repo, and they did not run when editing files in the UI or merging pull-requests.

We could exec these hooks during these actions from the Web UI, but that would make the behavior non-standard.
If we write code to exec scripts in a non-standard way, we could name the files in a gogs namespace such as ./hooks/gogs-post-merge or ./hooks/gogs/post-commit

cc: @unknwon / @xiayhub