git-p4: Allow branch definition with git config

Perforce does not strictly require the usage of branch specifications to create
branches. In these cases the branch detection code of git-p4 will not be able to
import them.
This patch adds support for git-p4.branchList configuration option, allowing
branches to be defined in git config.

Signed-off-by: Vitor Antunes <vitor.hda@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -342,6 +342,11 @@ def gitConfig(key, args = None): # set args to "--bool", for instance
         _gitConfig[key] = read_pipe(cmd, ignore_error=True).strip()
     return _gitConfig[key]
 
+def gitConfigList(key):
+    if not _gitConfig.has_key(key):
+        _gitConfig[key] = read_pipe("git config --get-all %s" % key, ignore_error=True).strip().split(os.linesep)
+    return _gitConfig[key]
+
 def p4BranchesInGit(branchesAreInRemotes = True):
     branches = {}
 
@@ -1490,6 +1495,25 @@ class P4Sync(Command, P4UserMap):
                     if source not in self.knownBranches:
                         lostAndFoundBranches.add(source)
 
+        # Perforce does not strictly require branches to be defined, so we also
+        # check git config for a branch list.
+        #
+        # Example of branch definition in git config file:
+        # [git-p4]
+        #   branchList=main:branchA
+        #   branchList=main:branchB
+        #   branchList=branchA:branchC
+        configBranches = gitConfigList("git-p4.branchList")
+        for branch in configBranches:
+            if branch:
+                (source, destination) = branch.split(":")
+                self.knownBranches[destination] = source
+
+                lostAndFoundBranches.discard(destination)
+
+                if source not in self.knownBranches:
+                    lostAndFoundBranches.add(source)
+
 
         for branch in lostAndFoundBranches:
             self.knownBranches[branch] = branch

contrib/fast-import/git-p4.txt

@@ -263,6 +263,13 @@ Only use branch specifications defined by the selected username.
 
   git config [--global] git-p4.branchUser username
 
+git-p4.branchList
+
+List of branches to be imported when branch detection is enabled.
+
+  git config [--global] git-p4.branchList main:branchA
+  git config [--global] --add git-p4.branchList main:branchB
+
 Implementation Details...
 =========================