pretty-formats: add 'format:<string>'

pretty-formats: add 'format:<string>'

With this patch,

$ git show -s \
	--pretty=format:'  Ze komit %h woss%n  dunn buy ze great %an'

shows something like

  Ze komit 04c5c88 woss
  dunn buy ze great Junio C Hamano

The supported placeholders are:

	'%H': commit hash
	'%h': abbreviated commit hash
	'%T': tree hash
	'%t': abbreviated tree hash
	'%P': parent hashes
	'%p': abbreviated parent hashes
	'%an': author name
	'%ae': author email
	'%ad': author date
	'%aD': author date, RFC2822 style
	'%ar': author date, relative
	'%at': author date, UNIX timestamp
	'%cn': committer name
	'%ce': committer email
	'%cd': committer date
	'%cD': committer date, RFC2822 style
	'%cr': committer date, relative
	'%ct': committer date, UNIX timestamp
	'%e': encoding
	'%s': subject
	'%b': body
	'%Cred': switch color to red
	'%Cgreen': switch color to green
	'%Cblue': switch color to blue
	'%Creset': reset color
	'%n': newline

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>