git的输出格式，可以极大的方便我们处理输出字符串
最好的还是查看官方手册，虽然长，但有用！
http://www.kernel.org/pub/software/scm/git/docs/git-show.html
[code lang="html"]
The 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
%aN: author name (respecting .mailmap, see git-shortlog(1) or git-blame(1))
%ae: author email
%aE: author email (respecting .mailmap, see git-shortlog(1) or git-blame(1))
%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 format
%cn: committer name
%cN: committer name (respecting .mailmap, see git-shortlog(1) or git-blame(1))
%ce: committer email
%cE: committer email (respecting .mailmap, see git-shortlog(1) or git-blame(1))
%cd: committer date
%cD: committer date, RFC2822 style
%cr: committer date, relative
%ct: committer date, UNIX timestamp
%ci: committer date, ISO 8601 format
%d: ref names, like the --decorate option of git-log(1)
%e: encoding
%s: subject
%f: sanitized subject line, suitable for a filename
%b: body
%B: raw body (unwrapped subject and body)
%N: commit notes
%gD: reflog selector, e.g., refs/stash@{1}
%gd: shortened reflog selector, e.g., stash@{1}
%gn: reflog identity name
%gN: reflog identity name (respecting .mailmap, see git-shortlog(1) or git-blame(1))
%ge: reflog identity email
%gE: reflog identity email (respecting .mailmap, see git-shortlog(1) or git-blame(1))
%gs: reflog subject
%Cred: switch color to red
%Cgreen: switch color to green
%Cblue: switch color to blue
%Creset: reset color
%C(…): color specification, as described in color.branch.* config option
%m: left, right or boundary mark
%n: newline
%%: a raw %
%x00: print a byte from a hex code
%w([[,[,]]]): switch line wrapping, like the -w option of git-shortlog(1).
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