默认git log 出来的格式并不是特别直观，很多时候想要更简便的输出更多或者更少的信息，这里列出几个git log的format。
可以根据自己的需要定制。
git log命令可一接受一个–pretty选项，来确定输出的格式.
比如 ：
如果我们只想输出hash.
git log --pretty=format:"%h"
详细 命令 ：
‘%H’: commit hash
‘%h’: abbreviated commit hash
‘%T’: tree hash
‘%t’: abbreviated tree hash
‘%P’: parent hashes
‘%p’: abbreviated parent hashes
‘%an’: author name
‘%aN’: author name (respecting .mailmap, see git-shortlog or git-blame)
‘%ae’: author email
‘%aE’: author email (respecting .mailmap, see git-shortlog or git-blame)
‘%ad’: author date (format respects --date= option)
‘%aD’: author date, RFC2822 style
‘%ar’: author date, relative
‘%at’: author date, UNIX timestamp
‘%ai’: author date, ISO 8601-like format
‘%aI’: author date, strict ISO 8601 format
‘%cn’: committer name
‘%cN’: committer name (respecting .mailmap, see git-shortlog or git-blame)
‘%ce’: committer email
‘%cE’: committer email (respecting .mailmap, see git-shortlog or git-blame)
‘%cd’: committer date (format respects --date= option)
‘%cD’: committer date, RFC2822 style
‘%cr’: committer date, relative
‘%ct’: committer date, UNIX timestamp
‘%ci’: committer date, ISO 8601-like format
‘%cI’: committer date, strict ISO 8601 format
‘%d’: ref names, like the --decorate option of git-log
‘%D’: ref names without the " (", “)” wrapping.
‘%e’: encoding
‘%s’: subject
‘%f’: sanitized subject line, suitable for a filename
‘%b’: body
‘%B’: raw body (unwrapped subject and body)
‘%N’: commit notes
‘%GG’: raw verification message from GPG for a signed commit