Fix slowness when searching for networked git repos under Cygwin.

The algorithm in fugitive#extract_git_dir() is to move upwards in the
file system hierarchy until a sub-directory called .git is found. When
accessing a file on a network share from a Cygwin Vim and the file is not
within a git repo, this eventually causes a check for the existence of
//serverName/.git and //.git. Such checks are extremely slow so let's
avoid them.
Add & modify autocmd events `FugitiveBoot` & `Fugitive`

- Change old `Fugitive` to `FugitiveBoot`

- Put `Fugitive` at the end of fugitive#detect
Reopen diff folds when diff ends

References #36.
Focus diff window on :Gdiff

References #36.
Add basic Bitbucket support for :Gbrowse
Fix :Gbrowse URLs for Bitbucket

* Change anchor format for links to line numbers
* Change URL for trees
* Change URL for commits
Show Bitbucket support in :Gbrowse documentation