tree: sync part httprouter codes and reduce if/else

[thinkerou]

the pr includes three parts:

1. use longestCommonPrefix from https://github.com/julienschmidt/httprouter/blob/master/tree.go#L20
2. remove else statement to reduce indent
3. sync the issue fixed wildcard conflict should panic but doesn't julienschmidt/httprouter#172

after:

➜  go-http-routing-benchmark git:(master) go test -bench=Gin
#GithubAPI Routes: 203
   Gin: 58512 Bytes

#GPlusAPI Routes: 13
   Gin: 4384 Bytes

#ParseAPI Routes: 26
   Gin: 7776 Bytes

#Static Routes: 157
   Gin: 34936 Bytes

goos: darwin
goarch: amd64
pkg: github.com/julienschmidt/go-http-routing-benchmark
BenchmarkGin_Param        	12134269	       100 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Param5       	 6822183	       173 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Param20      	 2442889	       479 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParamWrite   	 6708414	       168 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubStatic 	 9077402	       118 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubParam  	 5660514	       203 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubAll    	   26366	     43260 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusStatic  	13235361	        83.2 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusParam   	 9402622	       127 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlus2Params 	 6166275	       183 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusAll     	  577646	      1866 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseStatic  	12644926	        86.3 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseParam   	11198742	        99.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Parse2Params 	 9181192	       127 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseAll     	  319550	      3366 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_StaticAll    	   40864	     26833 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/julienschmidt/go-http-routing-benchmark	22.169s

before:

➜  go-http-routing-benchmark git:(master) go test -bench=Gin
#GithubAPI Routes: 203
   Gin: 58512 Bytes

#GPlusAPI Routes: 13
   Gin: 4384 Bytes

#ParseAPI Routes: 26
   Gin: 7776 Bytes

#Static Routes: 157
   Gin: 34936 Bytes

goos: darwin
goarch: amd64
pkg: github.com/julienschmidt/go-http-routing-benchmark
BenchmarkGin_Param        	10962327	       104 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Param5       	 6442416	       179 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Param20      	 2381566	       523 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParamWrite   	 6295077	       173 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubStatic 	 9231175	       130 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubParam  	 4975027	       219 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GithubAll    	   24982	     47799 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusStatic  	12289202	        96.7 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusParam   	 9156241	       138 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlus2Params 	 5860107	       210 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_GPlusAll     	  552486	      2111 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseStatic  	11597578	       105 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseParam   	 9847570	       113 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_Parse2Params 	 7823059	       142 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_ParseAll     	  290121	      3575 ns/op	       0 B/op	       0 allocs/op
BenchmarkGin_StaticAll    	   41952	     28302 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/julienschmidt/go-http-routing-benchmark	22.864s

[codecov]

Codecov Report

Merging #2163 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2163      +/-   ##
==========================================
+ Coverage   98.61%   98.61%   +<.01%     
==========================================
  Files          40       40              
  Lines        2240     2242       +2     
==========================================
+ Hits         2209     2211       +2     
  Misses         16       16              
  Partials       15       15

Impacted Files	Coverage Δ
tree.go	100% <100%> (ø)	⬆️

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