githelp: clean up reflog help This referenced commands that don't exist in core. The new help isn't great since it references an experimental extension. But it is better than nothing. While we're here, also add test coverage. Differential Revision: https://phab.mercurial-scm.org/D1727

githelp: replace suggestion of `hg record` `hg record` is deprecated in favor of `hg commit --interactive`. Differential Revision: https://phab.mercurial-scm.org/D1726

githelp: remove reference to tweakdefaults This is a Facebook-ism. Differential Revision: https://phab.mercurial-scm.org/D1725

githelp: recommend `hg import` for `git am` This referenced a third party extension. It doesn't feel appropriate to do that from a core extension. Reference `hg import` instead. Differential Revision: https://phab.mercurial-scm.org/D1724

githelp: improve help for `git add` The old code was referencing record and crecord, which are deprecated. `hg commit --interactive` is the preferred mechanism to use. In addition, there was duplicate code in this function. It has been removed. Tests have been added to cover this function. Differential Revision: https://phab.mercurial-scm.org/D1723

githelp: vendor Facebook authored extension This commit vendors the Facebook-authored "githelp" extension. This extension provides a "githelp" command that can be used to try to convert a `git` command to its Mercurial equivalent. This functionality is useful for Git users learning Mercurial. The extension was copied from the repository at revision 32ceeccb832c433b36e9af8196814b8e5a526775. The following modifications were made: * The "testedwith" value has been changed to match core's conventions. * Support for a custom footer has been removed, as it is Facebook specific. The feature is useful. But the implementation wasn't appropriate for core. * A test referencing "tweakdefaults" has been removed. * Imports changed to match Mercurial's style convention. * Double newlines in test removed. * Pager activation changed to ui.pager(). * Initial line of githelp.py changed to add description of file. The removal of the custom footer code was the only significant source change. The rest were mostly cosmetic. There are still some Facebook-isms in the extension. I'll address these as follow-ups. .. feature:: githelp extension The "githelp" extension provides the ``hg githelp`` command. This command attempts to convert a ``git`` command to its Mercurial equivalent. The extension can be useful to Git users new to Mercurial. Differential Revision: https://phab.mercurial-scm.org/D1722