Fix :Gblame width with custom date format

Partially addresses #242.
Fix <C-W><C-O> in diff below :Gstatus

I'll admit I can't remember the original purpose of this code.

Closes #66.
Return to correct window when closing Gblame window

Vim does not guarantee persistent window numbers. Instead, windows are
numbered according to their position on the screen, with the topmost,
leftmost window always having number 1, and the bottommost, rightmost
window always having a number equal to the total number of windows
currently visible. Crucially, this means that, when a window is closed,
windows which come "after" it in the positional order will be
renumbered.

When fugitive's Gblame window is closed, e.g. by pressing `q`, it
attempts to return focus to the window of the blamed buffer. Previously,
the number of the window to return to was computed before closing the
Gblame window, then the Gblame window was closed, then the blamed
buffer's window was focused. However, since windows were often
renumbered as soon as the Gblame window was closed, this would
frequently cause focus to jump to the window *after* the blamed buffer's
window, rather than the intended behavior.

This corrects the issue by jumping to the proper return window prior to
deleting the Gblame buffer, ensuring that the computed window number is
in fact correct at the moment when the focus change occurs.
Accept a count with :Gstatus <C-N>/<C-P>
Allow <C-p> pass through to ctrlp.vim in :Gstatus

This only works on the first line.  Any more than that, and we'll have
to give up on the whole <C-p> map entirely (which is not off the table).
Support ctrlp.vim in historical buffers
Pick sane git_dir for :Gedit from :Gstatus window

When using a :Gedit command from the :Gstatus window the git_dir was
being based on the window that was switched into in order to edit the
file. So if Fugitive switched into a window with a file from a different
Git repo (or a file with no Git repo) the :Gedit command could fail or
edit the wrong file.

Instead base the git_dir on the window from which the :Gedit command
originated.
s:Detect: only add readable files to &tags and doc

Fixes issue #104.
Fix A in :Gblame with unix timestamp
Allow a count to blame resizers

Pass a count to A or C to add that many additional columns, or to D to
remove that many additional columns.
Don't show diff in keywordprg output in GUI

It's not very readable without syntax highlighting or pagination, so
leave it off.
Update :Gblame supported options
Conceal file names and line numbers in :Gblame
Fix :Gbrowse when argument starts with a digit
Mention OS X browser in FAQ

Closes #255.
Correct foldtext when diff.mnemonic is set
Provide failsafe for foldtext filename

Closes #216.
Replace :Gstatus cv with cva and cvc

My rebuttal to #263.
Escape # in :Glog

Closes #265.
Allow :Gbrowse on HTTP GitHub FI

Closes #272.