Don't consider "tag-prefix" as optional if it's explicitly specified

To correctly calculate the next version with the prefix.

For example, we develop the main product and some its plugin
in the same repository and have the following git tags list:
- 0.1.0
- 0.1.1
- 0.2.0
- 0.2.1
- plugin_0.1.0
- plugin_0.1.1

The expected next plugin version is `0.1.2` not `0.2.2` (`plugin_` prefix
will be prepended before setting the tag).

Author: AlexPykavy

src/GitVersion.Core.Tests/Configuration/ConfigurationProviderTests.CanWriteOutEffectiveConfiguration.approved.txt

@@ -1,7 +1,7 @@
 assembly-versioning-scheme: MajorMinorPatch
 assembly-file-versioning-scheme: MajorMinorPatch
 mode: ContinuousDelivery
-tag-prefix: '[vV]'
+tag-prefix: '[vV]?'
 continuous-delivery-fallback-tag: ci
 major-version-bump-message: '\+semver:\s?(breaking|major)'
 minor-version-bump-message: '\+semver:\s?(feature|minor)'

src/GitVersion.Core/Configuration/GitVersionConfiguration.cs

@@ -97,7 +97,7 @@ public override string ToString()
         return stringBuilder.ToString();
     }
 
-    public const string DefaultTagPrefix = "[vV]";
+    public const string DefaultTagPrefix = "[vV]?";
     public const string ReleaseBranchRegex = "^releases?[/-]";
     public const string FeatureBranchRegex = "^features?[/-]";
     public const string PullRequestRegex = @"^(pull|pull\-requests|pr)[/-]";

src/GitVersion.Core/PublicAPI.Shipped.txt

@@ -28,7 +28,7 @@ const GitVersion.BuildAgents.SpaceAutomation.EnvironmentVariableName = "JB_SPACE
 const GitVersion.BuildAgents.TeamCity.EnvironmentVariableName = "TEAMCITY_VERSION" -> string!
 const GitVersion.BuildAgents.TravisCi.EnvironmentVariableName = "TRAVIS" -> string!
 const GitVersion.Configuration.ConfigurationFileLocator.DefaultFileName = "GitVersion.yml" -> string!
-const GitVersion.Configuration.GitVersionConfiguration.DefaultTagPrefix = "[vV]" -> string!
+const GitVersion.Configuration.GitVersionConfiguration.DefaultTagPrefix = "[vV]?" -> string!
 const GitVersion.Configuration.GitVersionConfiguration.DevelopBranchKey = "develop" -> string!
 const GitVersion.Configuration.GitVersionConfiguration.DevelopBranchRegex = "^dev(elop)?(ment)?$" -> string!
 const GitVersion.Configuration.GitVersionConfiguration.FeatureBranchKey = "feature" -> string!

src/GitVersion.Core/VersionCalculation/SemanticVersioning/SemanticVersion.cs

@@ -143,7 +143,7 @@ public static SemanticVersion Parse(string version, string? tagPrefixRegex, Sema
 
     public static bool TryParse(string version, string? tagPrefixRegex, [NotNullWhen(true)] out SemanticVersion? semanticVersion, SemanticVersionFormat format = SemanticVersionFormat.Strict)
     {
-        var match = Regex.Match(version, $"^({tagPrefixRegex})?(?<version>.*)$");
+        var match = Regex.Match(version, $"^({tagPrefixRegex})(?<version>.*)$");
 
         if (!match.Success)
         {