perf: avoid allocations with (*regexp.Regexp).MatchString (#3881)

We should use `(*regexp.Regexp).MatchString` instead of
`(*regexp.Regexp).Match([]byte(...))` when matching string to avoid
unnecessary `[]byte` conversions and reduce allocations.

Example benchmark:

func BenchmarkMatch(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := branchRegex.Match([]byte("main")); !match {
			b.Fail()
		}
	}
}

func BenchmarkMatchString(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := branchRegex.MatchString("main"); !match {
			b.Fail()
		}
	}
}

goos: linux
goarch: amd64
pkg: github.com/runatlantis/atlantis/server/core/config
cpu: AMD Ryzen 7 PRO 4750U with Radeon Graphics
BenchmarkMatch-16          	 8269699	       141.4 ns/op	       4 B/op	       1 allocs/op
BenchmarkMatchString-16    	14298446	        95.81 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/runatlantis/atlantis/server/core/config	2.784s

Signed-off-by: Eng Zer Jun <[email protected]>
Co-authored-by: PePe Amengual <[email protected]>

server/core/config/parser_validator.go

@@ -78,7 +78,7 @@ func (p *ParserValidator) ParseRepoCfgData(repoCfgData []byte, globalCfg valid.G
 	//
 	i := 0
 	for _, p := range validConfig.Projects {
-		if branch == "" || p.BranchRegex == nil || p.BranchRegex.Match([]byte(branch)) {
+		if branch == "" || p.BranchRegex == nil || p.BranchRegex.MatchString(branch) {
 			validConfig.Projects[i] = p
 			i++
 		}