Truncate object fields in Herald transcripts

Summary:
A few users have hit cases where Herald transcripts of large commits exceed the MySQL packet limit, because one of the fields in the transcript is an enomrous textual diff.

There's no value in saving these huge amounts of data. Transcripts are useful for understanding the action of Herald rules, but can be reconstructed later. Instead of saving all of the data, limit each field to 4KB of data.

For strings, we just truncate at 4KB. For arrays, we truncate after 4KB of values.

Test Plan: Ran unit tests. Artificially decreased limit and ran transcripts, saw them truncate properly.

Reviewers: btrahan

Reviewed By: btrahan

CC: frgtn, aran

Differential Revision: https://secure.phabricator.com/D7783
Remove many redundant implementations of `canLoadNamedObject()`

Summary:
These just got copy/pasted like crazy, the base class has the correct default implementation.

(I'm adding "H" for Herald Rules, which is why I was in this code.)

I also documented the existing prefixes at [[ Object Name Prefixes ]].

Test Plan: Verified base implementation. Typed some object names into the jump nav.

Reviewers: btrahan

Reviewed By: btrahan

CC: hach-que, aran

Differential Revision: https://secure.phabricator.com/D7785
Give Herald rules a standard "Hnnn" object name

Summary: Allow Herald rules to be referred to with `H123`, etc., like other object types are. Herald rules now have proper PHIDs and an increasingly prominent role in triggering application actions. Although I suspect users will rarely use `H123` in Remarkup to mention rules, this can simplify some of the interfaces which relate objects across systems.

Test Plan: Looked at various interfaces and saw `H123` names. Mentioned `H123` in remarkup.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7786
Provide basic commit content hooks for Herald

Summary: Ref T4195. This doesn't provide any interesting fields yet (content, affected paths, commit message) but fires the hook correctly.

Test Plan: Added a blocking hook and saw it fire.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7789
Support "changed filename" and "file content" fields for commit content Herald rules

Summary: Ref T4195. Adds support for diff content rules.

Test Plan: Pushed SVN and Git changes through, saw them generate reasonable transcripts. Mercurial still isn't hooked up to this phase.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7791
Support content pre-commit hooks in Mercurial

Summary: Ref T4195. Add Mercurial support to the content hook phase.

Test Plan:
Here are some `commit` push logs for a Mercurial repo:

{F90689}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7792
Get the proper Subversion URI for hosted repositories.

Summary: The commit change parser worker was incorrectly getting the remote uri; calling getSubversionPathURI instead behaves properly for hosted repositories.  Fixes T4236.

Test Plan: Observed that my hosted subversion repositories finally had history in Diffusion.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4236

Differential Revision: https://secure.phabricator.com/D7794