Hotfix for VerCraft CI usage (#19)

### What's done:
- All CI platforms do not checkout particular branch, but provide a variable, which contains the name of a particular branch;
- So VerCraft tried to get the checked-out branch, but was not able to do it;
- Now VerCraft uses this variable and also has `VERCRAFT_BRANCH_REF` for the branch name to be explicit set;

core/src/main/kotlin/com/akuleshov7/vercraft/core/Branch.kt

@@ -4,6 +4,11 @@ import org.eclipse.jgit.api.Git
 import org.eclipse.jgit.lib.Ref
 import org.eclipse.jgit.revwalk.RevCommit
 
+public const val GITLAB_BRANCH_REF: String = "CI_COMMIT_REF_NAME"
+public const val GITHUB_HEAD_REF: String = "GITHUB_HEAD_REF"
+public const val BITBUCKET_BRANCH_REF: String = "BITBUCKET_BRANCH"
+public const val VERCRAFT_BRANCH_REF: String = "VERCRAFT_BRANCH"
+
 public class Branch(git: Git, public val ref: Ref) {
     public val gitLog: List<RevCommit> = git.log().add(ref.objectId).call().toList()
 

core/src/main/kotlin/com/akuleshov7/vercraft/core/Releases.kt

@@ -52,7 +52,41 @@ public class Releases public constructor(private val git: Git) {
 
     public val releaseBranches: MutableSet<ReleaseBranch> = findReleaseBranches()
 
-    private val currentCheckoutBranch = Branch(git, repo.findRef(repo.branch))
+    private val currentCheckoutBranch = repo.branch
+        ?.let { Branch(git, repo.findRef(it)) }
+        ?: run {
+            logger.warn(
+                "$ERROR_PREFIX your current HEAD is detached (no branch is checked out). " +
+                        "Usually this happens on CI platforms, which check out particular commit. " +
+                        "Trying to resolve branch name using known CI ENV variables: " +
+                        "$GITLAB_BRANCH_REF, $GITHUB_HEAD_REF, $BITBUCKET_BRANCH_REF."
+            )
+
+            val branchName = System.getenv(GITLAB_BRANCH_REF)
+                ?: System.getenv(GITHUB_HEAD_REF)
+                ?: System.getenv(BITBUCKET_BRANCH_REF)
+                ?: System.getenv(VERCRAFT_BRANCH_REF)
+                ?: run {
+                    logger.warn(
+                        "$ERROR_PREFIX following variables are not defined in current env" +
+                                "$GITLAB_BRANCH_REF, $GITHUB_HEAD_REF, $BITBUCKET_BRANCH_REF" +
+                                "Please pass the branch name which you are trying to process now explicitly " +
+                                "to VerCraft by setting ENV variable \$VERCRAFT_BRANCH_REF. "
+                    )
+                    throw NullPointerException(
+                        "Current HEAD is detached and CI env variables with the branch name are not set, so" +
+                                "not able to determine the original branch name."
+                    )
+                }
+
+            Branch(
+                git,
+                git.branchList()
+                    .setListMode(REMOTE)
+                    .call()
+                    .first { it.name.endsWith(branchName) }
+            )
+        }
 
     public val version: VersionCalculator = VersionCalculator(git, this, currentCheckoutBranch)
 

core/src/main/kotlin/com/akuleshov7/vercraft/core/VersionCalculator.kt

@@ -127,7 +127,9 @@ public class VersionCalculator(
             dateFormat.timeZone = TimeZone.getDefault()
             val formattedDate = dateFormat.format(Date(commit.commitTime * 1000L))
 
-            return SemVer(NO_MAJOR, NO_MINOR, distance + 1).setPrefix("$formattedDate-$branch")
+            return SemVer(NO_MAJOR, NO_MINOR, distance + 1)
+                .setPrefix("$formattedDate-$branch")
+                .setPostFix(commit.name.substring(0, 5))
         }
     }